Initial commit: opencode_bridge JupyterLab extension (Slices 1-3.5)

A JupyterLab extension that bridges the cell UI to a local OpenCode Serve
process. The extension is a dual package: a Python server extension
exposed under /opencode-bridge/*, plus a TypeScript frontend that
registers per-cell toolbars.

Backend (Python, tornado)
- Slice 1: config + auth + OpenCode HTTP client (tornado.httpclient,
  no aiohttp). 4 settings in schema/plugin.json (url, user, password,
  request timeout).
- Slice 2: handlers for /hello, /health, /providers, /edit.
- Slice 2.1 (correction): SessionManager with 1 notebook = 1 session
  mapping, async-safe via per-path locks, 404 recovery via invalidate().
  Two new endpoints: GET /sessions, DELETE /session?notebook=<path>.
- 32 pytest tests pass.

Frontend (TypeScript, JupyterLab 4.6)
- src/types.ts: CellContext, OpenCodeRequest/Response, OpenCodeSettings.
- src/context/cell_context.ts: extract CellContext from a CodeCell +
  its parent NotebookPanel, structured error collection.
- src/api/opencode_client.ts: callOpenCodeEdit, callOpenCodeProviders.
- src/components/opencode_cell_footer.ts: OpenCodeCellFooter Widget
  implementing ICellFooter with 3 buttons (optimize / fix / edit),
  resolved via this.parent instanceof CodeCell. NOT cellToolbar
  (does not exist in JL 4.6) and NOT Widget.findParent (removed in
  @lumino/widgets 2.x).
- src/components/opencode_cell_factory.ts: Cell.ContentFactory
  subclass returning the OpenCodeCellFooter.
- src/components/opencode_installer.ts: installOpenCodeEverywhere
  patches every notebook (existing + new) to use the custom factory.
- src/index.ts: registers the factory, loads settings, fetches
  /providers on activation and logs the list to the console.
- 23 jest tests pass (mocked JupyterLab boundary, pnpm path safe).

Settings
- 6 fields: 3 auth (url/user/password) + 1 timeout + 2 model selection
  (provider/model). Provider list is fetched at startup from
  /opencode-bridge/providers and printed to the browser console so
  users can copy values into Settings Editor.

Docs
- design.md: 6 sections covering architecture, UI flow, API contract,
  TS skeletons, session management (v0.2.1 correction), and
  provider/model selection (v0.2.2 addition).
- CLAUDE.md: agent guidance for working in this repo.
- TODO.md: remaining work for Slices 4-7 + v0.4+ backlog.

CI
- Gitea release workflow at .github/workflows/build.yml.
- Bark notification helper (non-fatal on failure).

Generated artefacts ignored: opencode_bridge/labextension/, _version.py,
*.tsbuildinfo, junit.xml, test.ipynb scratch notebook.

CONTRIBUTING.md

@@ -0,0 +1,102 @@
+# Contributing
+
+## Development install
+
+Note: You will need Node.js to build the extension package.
+You may install it from [nodejs.org](https://nodejs.org/en/download). We
+recommend using the latest LTS version of Node.js.
+
+The `jlpm` command is JupyterLab's pinned version of
+[yarn](https://yarnpkg.com/) that is installed with JupyterLab. You may use
+`yarn` or `npm` in lieu of `jlpm` below.
+
+```bash
+# Clone the repo to your local environment
+# Change directory to the opencode_bridge directory
+
+# Set up a virtual environment and install package in development mode
+python -m venv .venv
+source .venv/bin/activate
+pip install --editable ".[dev,test]"
+
+# Link your development version of the extension with JupyterLab
+jupyter-builder develop . --overwrite
+# Server extension must be manually installed in develop mode
+jupyter server extension enable opencode_bridge
+
+# Rebuild extension Typescript source after making changes
+# IMPORTANT: Unlike the steps above which are performed only once, do this step
+# every time you make a change.
+jlpm build
+```
+
+You can watch the source directory and run JupyterLab at the same time in different terminals to watch for changes in the extension's source and automatically rebuild the extension.
+
+```bash
+# Watch the source directory in one terminal, automatically rebuilding when needed
+jlpm watch
+# Run JupyterLab in another terminal
+jupyter lab
+```
+
+With the watch command running, every saved change will immediately be built locally and available in your running JupyterLab. Refresh JupyterLab to load the change in your browser (you may need to wait several seconds for the extension to be rebuilt).
+
+By default, the `jlpm build` command generates the source maps for this extension to make it easier to debug using the browser dev tools. To also generate source maps for the JupyterLab core extensions, you can run the following command:
+
+```bash
+jupyter lab build --minimize=False
+```
+
+## Development uninstall
+
+```bash
+# Server extension must be manually disabled in develop mode
+jupyter server extension disable opencode_bridge
+pip uninstall opencode_bridge
+```
+
+In development mode, you will also need to remove the symlink created by `jupyter-builder develop`
+command. To find its location, you can run `jupyter labextension list` to figure out where the `labextensions`
+folder is located. Then you can remove the symlink named `opencode_bridge` within that folder.
+
+## Testing the extension
+
+### Server tests
+
+This extension is using [Pytest](https://docs.pytest.org/) for Python code testing.
+
+Install test dependencies (needed only once):
+
+```sh
+pip install -e ".[test]"
+# Each time you install the Python package, you need to restore the front-end extension link
+jupyter-builder develop . --overwrite
+```
+
+To execute them, run:
+
+```sh
+pytest -vv -r ap --cov opencode_bridge
+```
+
+#### Frontend tests
+
+This extension is using [Jest](https://jestjs.io/) for JavaScript code testing.
+
+To execute them, execute:
+
+```sh
+jlpm
+jlpm test
+```
+
+### Integration tests
+
+This extension uses [Playwright](https://playwright.dev/docs/intro) for the integration tests (aka user level tests).
+More precisely, the JupyterLab helper [Galata](https://github.com/jupyterlab/jupyterlab/tree/master/galata) is used to handle testing the extension in JupyterLab.
+
+More information is provided within the [ui-tests](./ui-tests/README.md) README.
+
+## Packaging the extension
+
+See [RELEASE](RELEASE.md)