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/base.py

@@ -0,0 +1,127 @@
+"""Abstract storage adapter for notebook snapshot backends.
+
+This module defines the common interface for dumb bytes buckets. It has no
+knowledge of notebooks, versions, manifests, compression, or hashing.
+"""
+
+from abc import ABC, abstractmethod
+
+
+def validate_key(key: str) -> str:
+    """Validate and return a POSIX-style relative storage key.
+
+    Rejects empty keys, absolute paths, parent-directory segments, and
+    backslashes. The returned value is the same string passed in.
+
+    Parameters
+    ----------
+    key: str
+        The storage key to validate.
+
+    Returns
+    -------
+    str
+        The validated key.
+
+    Raises
+    ------
+    ValueError
+        If the key is empty, absolute, contains ``..``, or contains
+        backslashes.
+    """
+    if not key:
+        raise ValueError("Storage key must not be empty")
+    if key.startswith("/"):
+        raise ValueError(f"Storage key must be relative, got absolute path: {key!r}")
+    if "\\" in key:
+        raise ValueError(f"Storage key must use POSIX separators, got backslash: {key!r}")
+    if ".." in key.split("/"):
+        raise ValueError(f"Storage key must not contain parent segments: {key!r}")
+    return key
+
+
+class StorageAdapter(ABC):
+    """Abstract base class for byte-oriented storage backends."""
+
+    @abstractmethod
+    def put(self, key: str, data: bytes) -> None:
+        """Store ``data`` under ``key``.
+
+        Parameters
+        ----------
+        key: str
+            The storage key.
+        data: bytes
+            The payload to store.
+        """
+        raise NotImplementedError
+
+    @abstractmethod
+    def get(self, key: str) -> bytes:
+        """Return the bytes stored under ``key``.
+
+        Parameters
+        ----------
+        key: str
+            The storage key.
+
+        Returns
+        -------
+        bytes
+            The stored payload.
+
+        Raises
+        ------
+        KeyError
+            If the key does not exist.
+        """
+        raise NotImplementedError
+
+    @abstractmethod
+    def list_keys(self, prefix: str = "") -> list[str]:
+        """Return a sorted list of keys starting with ``prefix``.
+
+        Parameters
+        ----------
+        prefix: str, optional
+            Filter keys to those beginning with this value. The empty string
+            matches all keys.
+
+        Returns
+        -------
+        list[str]
+            Sorted matching keys.
+        """
+        raise NotImplementedError
+
+    @abstractmethod
+    def delete(self, key: str) -> None:
+        """Delete the object at ``key``.
+
+        This method is idempotent: deleting a missing key does not raise.
+
+        Parameters
+        ----------
+        key: str
+            The storage key.
+        """
+        raise NotImplementedError
+
+    def exists(self, key: str) -> bool:
+        """Return whether ``key`` exists in storage.
+
+        Parameters
+        ----------
+        key: str
+            The storage key.
+
+        Returns
+        -------
+        bool
+            ``True`` if the key exists, ``False`` otherwise.
+        """
+        try:
+            self.get(key)
+            return True
+        except KeyError:
+            return False