dev: initial implementation of notebook-snapshot extension

Manual-commit notebook snapshot extension (JupyterLab 4):

Backend
- Storage adapter layer (Local + S3 via boto3, pip extra)
  - Dumb put/get/list/delete contract; key validation
  - S3Storage: lazy boto3 import, S3ConnectionError with friendly
    messages + connectivity check at startup
- SnapshotStore
  - Strip code outputs/execution_count, MD5 on cleaned JSON
  - Gzipped envelope (id/timestamp/name/description/hash/size/notebook)
  - Append-only manifest.json (rebuildable from version files)
  - SnapshotUnchangedError when new hash matches most recent version
- REST API (commit / list / content) under /snapshot/ namespace
- traitlets config (storage_type, local_root, s3_* + S3_* env fallback)

Frontend
- snapshot:commit command (toolbar button + command palette)
- Dialog for name + description
- SnapshotPanel (left sidebar timeline) + DiffWidget (main area)
- Cell diff: id-match first, LCS fallback; jsdiff for line diff
- Restore via model.fromJSON() + context.save() (no refresh)
- All AGENTS.md hard conventions enforced

Tests: 33 backend pytest passing (storage + store + routes)

Docs: AGENTS.md, design.md, README.md synced with implementation.

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/build.yml

@@ -0,0 +1,162 @@
+name: Build
+
+on:
+  push:
+    branches: main
+  pull_request:
+    branches: '*'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+
+    - name: Base Setup
+      uses: jupyterlab/maintainer-tools/.github/actions/base-setup@v1
+
+    - name: Install dependencies
+      run: python -m pip install -U "jupyterlab>=4.0.0,<5"
+
+    - name: Lint the extension
+      run: |
+        set -eux
+        jlpm
+        jlpm run lint:check
+
+    - name: Test the extension
+      run: |
+        set -eux
+        jlpm run test
+
+    - name: Build the extension
+      run: |
+        set -eux
+        python -m pip install .[test]
+
+        pytest -vv -r ap --cov snapshot
+        jupyter server extension list
+        jupyter server extension list 2>&1 | grep -ie "snapshot.*OK"
+
+        jupyter labextension list
+        jupyter labextension list 2>&1 | grep -ie "snapshot.*OK"
+        python -m jupyterlab.browser_check
+
+    - name: Package the extension
+      run: |
+        set -eux
+
+        pip install build
+        python -m build
+        pip uninstall -y "snapshot" jupyterlab
+
+    - name: Upload extension packages
+      uses: actions/upload-artifact@v4
+      with:
+        name: extension-artifacts
+        path: dist/snapshot*
+        if-no-files-found: error
+
+  test_isolated:
+    needs: build
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Install Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: '3.10'
+        architecture: 'x64'
+    - uses: actions/download-artifact@v4
+      with:
+        name: extension-artifacts
+    - name: Install and Test
+      run: |
+        set -eux
+        # Remove NodeJS, twice to take care of system and locally installed node versions.
+        sudo rm -rf $(which node)
+        sudo rm -rf $(which node)
+
+        pip install "jupyterlab>=4.0.0,<5" snapshot*.whl
+
+
+        jupyter server extension list
+        jupyter server extension list 2>&1 | grep -ie "snapshot.*OK"
+
+        jupyter labextension list
+        jupyter labextension list 2>&1 | grep -ie "snapshot.*OK"
+        python -m jupyterlab.browser_check --no-browser-test
+
+  integration-tests:
+    name: Integration tests
+    needs: build
+    runs-on: ubuntu-latest
+
+    env:
+      PLAYWRIGHT_BROWSERS_PATH: ${{ github.workspace }}/pw-browsers
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+
+    - name: Base Setup
+      uses: jupyterlab/maintainer-tools/.github/actions/base-setup@v1
+
+    - name: Download extension package
+      uses: actions/download-artifact@v4
+      with:
+        name: extension-artifacts
+
+    - name: Install the extension
+      run: |
+        set -eux
+        python -m pip install "jupyterlab>=4.0.0,<5" snapshot*.whl
+
+    - name: Install dependencies
+      working-directory: ui-tests
+      env:
+        YARN_ENABLE_IMMUTABLE_INSTALLS: 0
+        PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD: 1
+      run: jlpm install
+
+    - name: Set up browser cache
+      uses: actions/cache@v4
+      with:
+        path: |
+          ${{ github.workspace }}/pw-browsers
+        key: ${{ runner.os }}-${{ hashFiles('ui-tests/yarn.lock') }}
+
+    - name: Install browser
+      run: |
+        set -eux
+        jlpm playwright install chromium --only-shell
+      working-directory: ui-tests
+
+    - name: Execute integration tests
+      working-directory: ui-tests
+      run: |
+        jlpm playwright test
+
+    - name: Upload Playwright Test report
+      if: always()
+      uses: actions/upload-artifact@v4
+      with:
+        name: snapshot-playwright-tests
+        path: |
+          ui-tests/test-results
+          ui-tests/playwright-report
+
+  check_links:
+    name: Check Links
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - uses: actions/checkout@v4
+      - uses: jupyterlab/maintainer-tools/.github/actions/base-setup@v1
+      - uses: jupyterlab/maintainer-tools/.github/actions/check-links@v1

.github/workflows/check-release.yml

@@ -0,0 +1,30 @@
+name: Check Release
+on:
+  push:
+    branches: ["main"]
+  pull_request:
+    branches: ["*"]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  check_release:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Base Setup
+        uses: jupyterlab/maintainer-tools/.github/actions/base-setup@v1
+      - name: Check Release
+        uses: jupyter-server/jupyter_releaser/.github/actions/check-release@v2
+        with:
+
+          token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Upload Distributions
+        uses: actions/upload-artifact@v4
+        with:
+          name: snapshot-releaser-dist-${{ github.run_number }}
+          path: .jupyter_releaser_checkout/dist

.github/workflows/enforce-label.yml

@@ -0,0 +1,13 @@
+name: Enforce PR label
+
+on:
+  pull_request:
+    types: [labeled, unlabeled, opened, edited, synchronize]
+jobs:
+  enforce-label:
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+    steps:
+      - name: enforce-triage-label
+        uses: jupyterlab/maintainer-tools/.github/actions/enforce-label@v1

.github/workflows/prep-release.yml

@@ -0,0 +1,48 @@
+name: "Step 1: Prep Release"
+on:
+  workflow_dispatch:
+    inputs:
+      version_spec:
+        description: "New Version Specifier"
+        default: "next"
+        required: false
+      branch:
+        description: "The branch to target"
+        required: false
+      post_version_spec:
+        description: "Post Version Specifier"
+        required: false
+      # silent:
+      #   description: "Set a placeholder in the changelog and don't publish the release."
+      #   required: false
+      #   type: boolean
+      since:
+        description: "Use PRs with activity since this date or git reference"
+        required: false
+      since_last_stable:
+        description: "Use PRs with activity since the last stable git tag"
+        required: false
+        type: boolean
+jobs:
+  prep_release:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+    steps:
+      - uses: jupyterlab/maintainer-tools/.github/actions/base-setup@v1
+
+      - name: Prep Release
+        id: prep-release
+        uses: jupyter-server/jupyter_releaser/.github/actions/prep-release@v2
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          version_spec: ${{ github.event.inputs.version_spec }}
+          # silent: ${{ github.event.inputs.silent }}
+          post_version_spec: ${{ github.event.inputs.post_version_spec }}
+          branch: ${{ github.event.inputs.branch }}
+          since: ${{ github.event.inputs.since }}
+          since_last_stable: ${{ github.event.inputs.since_last_stable }}
+
+      - name: "** Next Step **"
+        run: |
+          echo "Optional): Review Draft Release: ${{ steps.prep-release.outputs.release_url }}"

.github/workflows/publish-release.yml

@@ -0,0 +1,58 @@
+name: "Step 2: Publish Release"
+on:
+  workflow_dispatch:
+    inputs:
+      branch:
+        description: "The target branch"
+        required: false
+      release_url:
+        description: "The URL of the draft GitHub release"
+        required: false
+      steps_to_skip:
+        description: "Comma separated list of steps to skip"
+        required: false
+
+jobs:
+  publish_release:
+    runs-on: ubuntu-latest
+    environment: release
+    permissions:
+      id-token: write
+    steps:
+      - uses: jupyterlab/maintainer-tools/.github/actions/base-setup@v1
+
+      - uses: actions/create-github-app-token@v1
+        id: app-token
+        with:
+          app-id: ${{ vars.APP_ID }}
+          private-key: ${{ secrets.APP_PRIVATE_KEY }}
+
+      - name: Populate Release
+        id: populate-release
+        uses: jupyter-server/jupyter_releaser/.github/actions/populate-release@v2
+        with:
+          token: ${{ steps.app-token.outputs.token }}
+          branch: ${{ github.event.inputs.branch }}
+          release_url: ${{ github.event.inputs.release_url }}
+          steps_to_skip: ${{ github.event.inputs.steps_to_skip }}
+
+      - name: Finalize Release
+        id: finalize-release
+        env:
+          NPM_TOKEN: ${{ secrets.NPM_TOKEN }}
+        uses: jupyter-server/jupyter_releaser/.github/actions/finalize-release@v2
+        with:
+          token: ${{ steps.app-token.outputs.token }}
+          release_url: ${{ steps.populate-release.outputs.release_url }}
+
+      - name: "** Next Step **"
+        if: ${{ success() }}
+        run: |
+          echo "Verify the final release"
+          echo ${{ steps.finalize-release.outputs.release_url }}
+
+      - name: "** Failure Message **"
+        if: ${{ failure() }}
+        run: |
+          echo "Failed to Publish the Draft Release Url:"
+          echo ${{ steps.populate-release.outputs.release_url }}

.github/workflows/update-integration-tests.yml

@@ -0,0 +1,39 @@
+name: Update Playwright Snapshots
+
+on:
+  issue_comment:
+    types: [created, edited]
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  update-snapshots:
+    runs-on: ubuntu-latest
+    if: github.event.issue.pull_request && contains(github.event.comment.body, 'please update snapshots')
+
+    steps:
+      - uses: jupyterlab/maintainer-tools/.github/actions/update-snapshots-checkout@main
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Base Setup
+        uses: jupyterlab/maintainer-tools/.github/actions/base-setup@v1
+
+      - name: Install dependencies
+        run: python -m pip install -U "jupyterlab>=4.0.0,<5"
+
+      - name: Install extension
+        run: |
+          set -eux
+          jlpm
+          python -m pip install .
+
+      - uses: jupyterlab/maintainer-tools/.github/actions/update-snapshots@v1
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          # Playwright knows how to start JupyterLab server
+          start_server_script: 'null'
+          test_folder: ui-tests
+          npm_client: jlpm