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>

ui-tests/README.md

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+# Integration Testing
+
+This folder contains the integration tests of the extension.
+
+They are defined using [Playwright](https://playwright.dev/docs/intro) test runner
+and [Galata](https://github.com/jupyterlab/jupyterlab/tree/main/galata) helper.
+
+The Playwright configuration is defined in [playwright.config.js](./playwright.config.js).
+
+The JupyterLab server configuration to use for the integration test is defined
+in [jupyter_server_test_config.py](./jupyter_server_test_config.py).
+
+The default configuration will produce video for failing tests and an HTML report.
+
+> There is a UI mode that you may like; see [that video](https://www.youtube.com/watch?v=jF0yA-JLQW0).
+
+## Run the tests
+
+> All commands are assumed to be executed from the root directory
+
+To run the tests, you need to:
+
+1. Compile the extension:
+
+```sh
+jlpm install
+jlpm build:prod
+```
+
+> Check the extension is installed in JupyterLab.
+
+2. Install test dependencies (needed only once):
+
+```sh
+cd ./ui-tests
+jlpm install
+jlpm playwright install
+cd ..
+```
+
+3. Execute the [Playwright](https://playwright.dev/docs/intro) tests:
+
+```sh
+cd ./ui-tests
+jlpm playwright test
+```
+
+Test results will be shown in the terminal. In case of any test failures, the test report
+will be opened in your browser at the end of the tests execution; see
+[Playwright documentation](https://playwright.dev/docs/test-reporters#html-reporter)
+for configuring that behavior.
+
+## Update the tests snapshots
+
+> All commands are assumed to be executed from the root directory
+
+If you are comparing snapshots to validate your tests, you may need to update
+the reference snapshots stored in the repository. To do that, you need to:
+
+1. Compile the extension:
+
+```sh
+jlpm install
+jlpm build:prod
+```
+
+> Check the extension is installed in JupyterLab.
+
+2. Install test dependencies (needed only once):
+
+```sh
+cd ./ui-tests
+jlpm install
+jlpm playwright install
+cd ..
+```
+
+3. Execute the [Playwright](https://playwright.dev/docs/intro) command:
+
+```sh
+cd ./ui-tests
+jlpm playwright test -u
+```
+
+> Some discrepancy may occurs between the snapshots generated on your computer and
+> the one generated on the CI. To ease updating the snapshots on a PR, you can
+> type `please update playwright snapshots` to trigger the update by a bot on the CI.
+> Once the bot has computed new snapshots, it will commit them to the PR branch.
+
+## Create tests
+
+> All commands are assumed to be executed from the root directory
+
+To create tests, the easiest way is to use the code generator tool of playwright:
+
+1. Compile the extension:
+
+```sh
+jlpm install
+jlpm build:prod
+```
+
+> Check the extension is installed in JupyterLab.
+
+2. Install test dependencies (needed only once):
+
+```sh
+cd ./ui-tests
+jlpm install
+jlpm playwright install
+cd ..
+```
+
+3. Start the server:
+
+```sh
+cd ./ui-tests
+jlpm start
+```
+
+4. Execute the [Playwright code generator](https://playwright.dev/docs/codegen) in **another terminal**:
+
+```sh
+cd ./ui-tests
+jlpm playwright codegen localhost:8888
+```
+
+## Debug tests
+
+> All commands are assumed to be executed from the root directory
+
+To debug tests, a good way is to use the inspector tool of playwright:
+
+1. Compile the extension:
+
+```sh
+jlpm install
+jlpm build:prod
+```
+
+> Check the extension is installed in JupyterLab.
+
+2. Install test dependencies (needed only once):
+
+```sh
+cd ./ui-tests
+jlpm install
+jlpm playwright install
+cd ..
+```
+
+3. Execute the Playwright tests in [debug mode](https://playwright.dev/docs/debug):
+
+```sh
+cd ./ui-tests
+jlpm playwright test --debug
+```
+
+## Upgrade Playwright and the browsers
+
+To update the web browser versions, you must update the package `@playwright/test`:
+
+```sh
+cd ./ui-tests
+jlpm up "@playwright/test"
+jlpm playwright install
+```

ui-tests/jupyter_server_test_config.py

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+"""Server configuration for integration tests.
+
+!! Never use this configuration in production because it
+opens the server to the world and provide access to JupyterLab
+JavaScript objects through the global window variable.
+"""
+from jupyterlab.galata import configure_jupyter_server
+
+configure_jupyter_server(c)
+
+# Uncomment to set server log level to debug level
+# c.ServerApp.log_level = "DEBUG"

ui-tests/package.json

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+{
+  "name": "snapshot-ui-tests",
+  "version": "1.0.0",
+  "description": "JupyterLab snapshot Integration Tests",
+  "private": true,
+  "scripts": {
+    "start": "jupyter lab --config jupyter_server_test_config.py",
+    "test": "jlpm playwright test",
+    "test:update": "jlpm playwright test --update-snapshots"
+  },
+  "devDependencies": {
+    "@jupyterlab/galata": "^5.0.5",
+    "@playwright/test": "^1.60.0"
+  }
+}

ui-tests/playwright.config.js

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+/**
+ * Configuration for Playwright using default from @jupyterlab/galata
+ */
+const baseConfig = require('@jupyterlab/galata/lib/playwright-config');
+
+module.exports = {
+  ...baseConfig,
+  webServer: {
+    command: 'jlpm start',
+    url: 'http://localhost:8888/lab',
+    timeout: 120 * 1000,
+    reuseExistingServer: !process.env.CI
+  }
+};

ui-tests/tests/snapshot.spec.ts

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+import { expect, test } from '@jupyterlab/galata';
+
+/**
+ * Don't load JupyterLab webpage before running the tests.
+ * This is required to ensure we capture all log messages.
+ */
+test.use({ autoGoto: false });
+
+test('should emit an activation console message', async ({ page }) => {
+  const logs: string[] = [];
+
+  page.on('console', message => {
+    logs.push(message.text());
+  });
+
+  await page.goto();
+
+  expect(
+    logs.filter(s => s === 'JupyterLab extension snapshot is activated!')
+  ).toHaveLength(1);
+});