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/tests/test_routes_api.py

@@ -0,0 +1,152 @@
+"""Integration tests for the snapshot REST API (commit / list / content)."""
+
+import json
+
+import pytest
+
+SAMPLE_NB = {
+    "cells": [
+        {
+            "cell_type": "code",
+            "id": "c1",
+            "source": "print('hi')",
+            "outputs": [{"output_type": "stream", "text": "hi"}],
+            "execution_count": 1,
+            "metadata": {},
+        },
+        {
+            "cell_type": "markdown",
+            "id": "c2",
+            "source": "# title",
+            "metadata": {},
+        },
+    ],
+    "metadata": {},
+    "nbformat": 4,
+    "nbformat_minor": 5,
+}
+
+
+def _post_commit(jp_fetch, path: str, name: str, description: str = ""):
+    body = json.dumps(
+        {
+            "path": path,
+            "content": SAMPLE_NB,
+            "name": name,
+            "description": description,
+        }
+    ).encode()
+    return jp_fetch(
+        "snapshot",
+        "notebook-version",
+        "commit",
+        method="POST",
+        body=body,
+    )
+
+
+async def test_commit_creates_entry(jp_fetch):
+    response = await _post_commit(jp_fetch, "api_a.ipynb", "v1", "first")
+    assert response.code == 200
+    payload = json.loads(response.body)
+    assert payload["entry"]["name"] == "v1"
+    assert payload["entry"]["description"] == "first"
+    assert payload["entry"]["id"].startswith("v_")
+    assert payload["entry"]["size"] > 0
+    assert payload["entry"]["hash"]
+
+
+async def test_list_contains_committed_entry(jp_fetch):
+    await _post_commit(jp_fetch, "api_b.ipynb", "v1", "desc")
+    response = await jp_fetch(
+        "snapshot",
+        "notebook-version",
+        "list",
+        params={"path": "api_b.ipynb"},
+    )
+    assert response.code == 200
+    payload = json.loads(response.body)
+    assert len(payload["versions"]) == 1
+    assert payload["versions"][0]["name"] == "v1"
+    assert payload["versions"][0]["description"] == "desc"
+
+
+async def test_content_returns_stripped_notebook(jp_fetch):
+    commit_resp = await _post_commit(jp_fetch, "api_c.ipynb", "v1")
+    entry = json.loads(commit_resp.body)["entry"]
+    response = await jp_fetch(
+        "snapshot",
+        "notebook-version",
+        "content",
+        params={"path": "api_c.ipynb", "id": entry["id"]},
+    )
+    assert response.code == 200
+    notebook = json.loads(response.body)
+    code = next(c for c in notebook["cells"] if c["cell_type"] == "code")
+    assert code["outputs"] == []
+    assert code["execution_count"] is None
+    assert notebook["nbformat"] == 4
+
+
+async def test_commit_missing_name_returns_400(jp_fetch):
+    body = json.dumps(
+        {"path": "api_d.ipynb", "content": SAMPLE_NB}
+    ).encode()
+    response = await jp_fetch(
+        "snapshot",
+        "notebook-version",
+        "commit",
+        method="POST",
+        body=body,
+        raise_error=False,
+    )
+    assert response.code == 400
+
+
+async def test_list_unknown_path_returns_empty(jp_fetch):
+    response = await jp_fetch(
+        "snapshot",
+        "notebook-version",
+        "list",
+        params={"path": "never_committed_unique.ipynb"},
+    )
+    assert response.code == 200
+    payload = json.loads(response.body)
+    assert payload["versions"] == []
+
+
+async def test_commit_duplicate_returns_skipped(jp_fetch):
+    """Second commit with identical content must NOT save; response is {skipped: True}."""
+    r1 = await _post_commit(jp_fetch, "api_dup.ipynb", "v1")
+    assert r1.code == 200
+    first = json.loads(r1.body)
+    assert first.get("entry") is not None
+    assert first.get("skipped") in (None, False)
+
+    r2 = await _post_commit(jp_fetch, "api_dup.ipynb", "v2")
+    assert r2.code == 200  # not an error — just a warning
+    second = json.loads(r2.body)
+    assert second.get("skipped") is True
+    assert second.get("reason") == "unchanged"
+    assert "未变更" in second.get("message", "") or "unchanged" in second.get("message", "").lower()
+    assert second.get("entry") is None
+
+    # And the manifest still has exactly one entry.
+    list_resp = await jp_fetch(
+        "snapshot",
+        "notebook-version",
+        "list",
+        params={"path": "api_dup.ipynb"},
+    )
+    assert json.loads(list_resp.body)["versions"].__len__() == 1
+
+
+async def test_content_unknown_id_returns_404(jp_fetch):
+    response = await jp_fetch(
+        "snapshot",
+        "notebook-version",
+        "content",
+        params={"path": "api_e.ipynb", "id": "v_0_doesnotexist"},
+        raise_error=False,
+    )
+    assert response.code == 404