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>

snapshot/storage/__init__.py

@@ -0,0 +1,68 @@
+"""Storage backends and factory for notebook snapshots."""
+
+from pathlib import Path
+
+from .base import StorageAdapter, validate_key
+from .local import LocalFileStorage
+from .s3 import S3ConnectionError, S3Storage
+
+
+def create_storage(
+    storage_type: str,
+    root_dir: str | Path,
+    **kwargs,
+) -> StorageAdapter:
+    """Create a storage adapter from a configuration string.
+
+    Parameters
+    ----------
+    storage_type: str
+        One of ``"local"`` or ``"s3"`` (covers AWS S3, MinIO, and any
+        S3-API-compatible service via the ``endpoint`` parameter).
+    root_dir: str | Path
+        Root directory used by local storage. For S3 this is ignored.
+    **kwargs
+        Extra arguments forwarded to the backend constructor. For S3,
+        expected keys include ``endpoint``, ``access_key``, ``secret_key``,
+        ``bucket``, ``secure``, and ``prefix``. For local, ``local_root`` may
+        override the default ``root_dir / ".notebook_versions"`` path.
+
+    Returns
+    -------
+    StorageAdapter
+        Configured storage adapter instance.
+
+    Raises
+    ------
+    ValueError
+        If ``storage_type`` is unknown.
+    S3ConnectionError
+        If ``storage_type="s3"`` and the endpoint is unreachable or
+        credentials are missing/invalid.
+    """
+    storage_type = storage_type.lower()
+    if storage_type == "local":
+        local_root = kwargs.get("local_root")
+        if local_root:
+            return LocalFileStorage(local_root)
+        return LocalFileStorage(Path(root_dir) / ".notebook_versions")
+    if storage_type == "s3":
+        return S3Storage(
+            endpoint=kwargs["endpoint"],
+            access_key=kwargs["access_key"],
+            secret_key=kwargs["secret_key"],
+            bucket=kwargs.get("bucket", "notebook-snapshot"),
+            secure=kwargs.get("secure", False),
+            prefix=kwargs.get("prefix", ""),
+        )
+    raise ValueError(f"Unknown storage type: {storage_type!r}")
+
+
+__all__ = [
+    "create_storage",
+    "LocalFileStorage",
+    "S3ConnectionError",
+    "S3Storage",
+    "StorageAdapter",
+    "validate_key",
+]