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>

eslint.config.mjs

@@ -0,0 +1,68 @@
+import js from '@eslint/js';
+import { defineConfig } from 'eslint/config';
+import tseslint from 'typescript-eslint';
+import prettierRecommended from 'eslint-plugin-prettier/recommended';
+import globals from 'globals';
+import jupyterPlugin from '@jupyter/eslint-plugin';
+
+export default defineConfig([
+  {
+    ignores: [
+      'node_modules',
+      'dist',
+      'coverage',
+      '**/*.js',
+      '**/*.d.ts',
+      '.venv',
+      'tests',
+      '**/__tests__',
+      'ui-tests'
+    ]
+  },
+  js.configs.recommended,
+  tseslint.configs.recommended,
+  jupyterPlugin.configs.recommended,
+  {
+    files: ['**/*.ts', '**/*.tsx'],
+    plugins: {
+      jupyter: jupyterPlugin
+    },
+    languageOptions: {
+      globals: {
+        ...globals.browser,
+        ...globals.es2015,
+        ...globals.node
+      },
+      parserOptions: {
+        project: 'tsconfig.json',
+        sourceType: 'module'
+      }
+    },
+    rules: {
+      '@typescript-eslint/naming-convention': [
+        'error',
+        {
+          selector: 'interface',
+          format: ['PascalCase'],
+          custom: {
+            regex: '^I[A-Z]',
+            match: true
+          }
+        }
+      ],
+      '@typescript-eslint/no-unused-vars': ['warn', { args: 'none' }],
+      '@typescript-eslint/no-explicit-any': 'off',
+      '@typescript-eslint/no-namespace': 'off',
+      '@typescript-eslint/no-use-before-define': 'off',
+      '@typescript-eslint/quotes': [
+        'error',
+        'single',
+        { avoidEscape: true, allowTemplateLiterals: false }
+      ],
+      curly: ['error', 'all'],
+      eqeqeq: 'error',
+      'prefer-arrow-callback': 'error'
+    }
+  },
+  prettierRecommended
+]);