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/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 }}"