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