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>

.gitignore

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+*.bundle.*
+lib/
+node_modules/
+*.log
+.eslintcache
+.stylelintcache
+*.egg-info/
+.ipynb_checkpoints
+*.tsbuildinfo
+snapshot/labextension
+# Version file is handled by hatchling
+snapshot/_version.py
+
+# IDE
+.idea/
+.vscode/
+
+# Local dev / test data
+data/
+test.ipynb
+
+# Integration tests
+ui-tests/test-results/
+ui-tests/playwright-report/
+
+# Created by https://www.gitignore.io/api/python
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+
+### Python ###
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+.venv
+
+# Byte-compiled / optimized / DLL files
+__pycache__/
+*.py[cod]
+*$py.class
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+# C extensions
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+# Distribution / packaging
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+*.egg
+MANIFEST
+
+# PyInstaller
+#  Usually these files are written by a python script from a template
+#  before PyInstaller builds the exe, so as to inject date/other infos into it.
+*.manifest
+*.spec
+
+# Installer logs
+pip-log.txt
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+# Unit test / coverage reports
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+*.cover
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+# Translations
+*.mo
+*.pot
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+# Scrapy stuff:
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+# Sphinx documentation
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+# PyBuilder
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+# pyenv
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+# celery beat schedule file
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+# SageMath parsed files
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+# Spyder project settings
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+# Rope project settings
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+# mkdocs documentation
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