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>

ui-tests/tests/snapshot.spec.ts

@@ -0,0 +1,21 @@
+import { expect, test } from '@jupyterlab/galata';
+
+/**
+ * Don't load JupyterLab webpage before running the tests.
+ * This is required to ensure we capture all log messages.
+ */
+test.use({ autoGoto: false });
+
+test('should emit an activation console message', async ({ page }) => {
+  const logs: string[] = [];
+
+  page.on('console', message => {
+    logs.push(message.text());
+  });
+
+  await page.goto();
+
+  expect(
+    logs.filter(s => s === 'JupyterLab extension snapshot is activated!')
+  ).toHaveLength(1);
+});