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>

pyproject.toml

@@ -0,0 +1,99 @@
+[build-system]
+requires = ["hatchling>=1.5.0", "hatch-nodejs-version>=0.3.2", "jupyter-builder>=1.0.0,<2"]
+build-backend = "hatchling.build"
+
+[project]
+name = "snapshot"
+readme = "README.md"
+license = { file = "LICENSE" }
+requires-python = ">=3.10"
+classifiers = [
+    "Framework :: Jupyter",
+    "Framework :: Jupyter :: JupyterLab",
+    "Framework :: Jupyter :: JupyterLab :: 4",
+    "Framework :: Jupyter :: JupyterLab :: Extensions",
+    "Framework :: Jupyter :: JupyterLab :: Extensions :: Prebuilt",
+    "License :: OSI Approved :: BSD License",
+    "Programming Language :: Python",
+    "Programming Language :: Python :: 3",
+    "Programming Language :: Python :: 3.10",
+    "Programming Language :: Python :: 3.11",
+    "Programming Language :: Python :: 3.12",
+    "Programming Language :: Python :: 3.13",
+    "Programming Language :: Python :: 3.14",
+]
+dependencies = [
+    "jupyter_server>=2.4.0,<3",
+]
+dynamic = ["version", "description", "authors", "urls", "keywords"]
+
+[project.optional-dependencies]
+dev = [
+    "jupyterlab>=4",
+    "jupyter-builder>=1.0.0",
+]
+test = [
+    "coverage",
+    "pytest",
+    "pytest-asyncio",
+    "pytest-cov",
+    "pytest-jupyter[server]>=0.6.0"
+]
+s3 = [
+    "boto3>=1.34.0",
+]
+
+[[tool.uv.index]]
+url = "https://pypi.tuna.tsinghua.edu.cn/simple/"
+default = true
+
+[tool.hatch.version]
+source = "nodejs"
+
+[tool.hatch.metadata.hooks.nodejs]
+fields = ["description", "authors", "urls", "keywords"]
+
+[tool.hatch.build.targets.sdist]
+artifacts = ["snapshot/labextension"]
+exclude = [".github", "binder"]
+
+[tool.hatch.build.targets.wheel.shared-data]
+"snapshot/labextension" = "share/jupyter/labextensions/snapshot"
+"install.json" = "share/jupyter/labextensions/snapshot/install.json"
+"jupyter-config/server-config" = "etc/jupyter/jupyter_server_config.d"
+
+[tool.hatch.build.hooks.version]
+path = "snapshot/_version.py"
+
+[tool.hatch.build.hooks.jupyter-builder]
+dependencies = ["hatch-jupyter-builder>=0.5"]
+build-function = "hatch_jupyter_builder.npm_builder"
+ensured-targets = [
+    "snapshot/labextension/static/style.js",
+    "snapshot/labextension/package.json",
+]
+skip-if-exists = ["snapshot/labextension/static/style.js"]
+
+[tool.hatch.build.hooks.jupyter-builder.build-kwargs]
+build_cmd = "build:prod"
+npm = ["jlpm"]
+
+[tool.hatch.build.hooks.jupyter-builder.editable-build-kwargs]
+build_cmd = "install:extension"
+npm = ["jlpm"]
+source_dir = "src"
+build_dir = "snapshot/labextension"
+
+[tool.jupyter-releaser.options]
+version_cmd = "hatch version"
+
+[tool.jupyter-releaser.hooks]
+before-build-npm = [
+    "python -m pip install 'jupyter-builder>=1.0.0,<2'",
+    "jlpm",
+    "jlpm build:prod"
+]
+before-build-python = ["jlpm clean:all"]
+
+[tool.check-wheel-contents]
+ignore = ["W002"]