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/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