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/__init__.py

@@ -0,0 +1,64 @@
+try:
+    from ._version import __version__
+except ImportError:
+    # Fallback when using the package in dev mode without installing
+    # in editable mode with pip. It is highly recommended to install
+    # the package from a stable release or in editable mode: https://pip.pypa.io/en/stable/topics/local-project-installs/#editable-installs
+    import warnings
+    warnings.warn("Importing 'snapshot' outside a proper installation.")
+    __version__ = "dev"
+from .config import SnapshotConfig
+from .routes import setup_route_handlers
+from .storage import create_storage
+from .store import SnapshotStore
+
+
+def _jupyter_labextension_paths():
+    return [{
+        "src": "labextension",
+        "dest": "snapshot"
+    }]
+
+
+def _jupyter_server_extension_points():
+    return [{
+        "module": "snapshot"
+    }]
+
+
+def _load_jupyter_server_extension(server_app):
+    """Registers the API handler to receive HTTP requests from the frontend extension.
+
+    Parameters
+    ----------
+    server_app: jupyterlab.labapp.LabApp
+        JupyterLab application instance
+    """
+    config = SnapshotConfig(config=server_app.config)
+    storage = create_storage(
+        storage_type=config.storage_type,
+        root_dir=server_app.root_dir,
+        local_root=config.local_root,
+        endpoint=config.s3_endpoint,
+        access_key=config.s3_access_key,
+        secret_key=config.s3_secret_key,
+        bucket=config.s3_bucket,
+        secure=config.s3_secure,
+        prefix=config.s3_prefix,
+    )
+    server_app.web_app.settings["snapshot_config"] = config
+    server_app.web_app.settings["snapshot_storage"] = storage
+    server_app.web_app.settings["snapshot_store"] = SnapshotStore(storage)
+    if config.storage_type == "s3":
+        server_app.log.info(
+            f"Snapshot S3 storage ready: endpoint={config.s3_endpoint}, "
+            f"bucket={config.s3_bucket}, secure={config.s3_secure}"
+        )
+    else:
+        server_app.log.info(
+            f"Activated snapshot storage backend: {config.storage_type}"
+        )
+
+    setup_route_handlers(server_app.web_app)
+    name = "snapshot"
+    server_app.log.info(f"Registered {name} server extension")