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/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 }}