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>

src/api.ts

@@ -0,0 +1,54 @@
+import { ServerConnection } from '@jupyterlab/services';
+
+import { requestAPI } from './request';
+
+export interface VersionEntry {
+  id: string;
+  timestamp: number;
+  name: string;
+  description: string;
+  hash: string;
+  size: number;
+}
+
+export interface CommitResponse {
+  entry: VersionEntry | null;
+  skipped: boolean;
+  reason: string | null;
+  message: string | null;
+}
+
+export async function commitSnapshot(
+  serverSettings: ServerConnection.ISettings,
+  path: string,
+  content: unknown,
+  name: string,
+  description: string
+): Promise<CommitResponse> {
+  return requestAPI<CommitResponse>('notebook-version/commit', serverSettings, {
+    method: 'POST',
+    body: JSON.stringify({ path, content, name, description }),
+    headers: { 'Content-Type': 'application/json' }
+  });
+}
+
+export async function listSnapshots(
+  serverSettings: ServerConnection.ISettings,
+  path: string
+): Promise<VersionEntry[]> {
+  const endpoint = `notebook-version/list?path=${encodeURIComponent(path)}`;
+  const response = await requestAPI<{ versions: VersionEntry[] }>(
+    endpoint,
+    serverSettings
+  );
+  return response.versions;
+}
+
+export async function getSnapshotContent(
+  serverSettings: ServerConnection.ISettings,
+  path: string,
+  id: string
+): Promise<unknown> {
+  const endpoint = `notebook-version/content?path=${encodeURIComponent(path)}&id=${encodeURIComponent(id)}`;
+  return requestAPI<unknown>(endpoint, serverSettings);
+}