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>

.copier-answers.yml

@@ -0,0 +1,19 @@
+# Changes here will be overwritten by Copier; NEVER EDIT MANUALLY
+_commit: v4.6.2
+_src_path: https://github.com/jupyterlab/extension-template
+advanced: true
+author_email: ''
+author_name: taochen
+create_claude_symlink: true
+create_gemini_symlink: false
+has_ai_rules: true
+has_binder: false
+has_settings: true
+kind: frontend-and-server
+labextension_name: snapshot
+project_short_description: A JupyterLab extension.
+python_name: snapshot
+repository: http://101.43.40.124:3000/tao.chen/notebook-snapshot-extension.git
+test: true
+yarn_linker: pnpm
+