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.

src/context/cell_context.ts

@@ -0,0 +1,81 @@
+/**
+ * Extract a CellContext snapshot from a CodeCell + its parent NotebookPanel.
+ *
+ * Pure functions, no DOM, no signals — testable with plain object mocks.
+ */
+import type { CodeCell, ICodeCellModel } from '@jupyterlab/cells';
+import type { NotebookPanel } from '@jupyterlab/notebook';
+
+import type { CellContext, ErrorOutput } from '../types';
+
+/**
+ * Find the first error output in a code cell's outputs.
+ * Returns null if the cell has no error output or is not a code cell.
+ */
+export function collectErrorOutputs(model: ICodeCellModel): ErrorOutput | null {
+  const outputs = model.outputs;
+  for (let i = 0; i < outputs.length; i++) {
+    const out = outputs.get(i);
+    if (out && out.type === 'error') {
+      const e = out as {
+        ename?: unknown;
+        evalue?: unknown;
+        traceback?: unknown;
+      };
+      return {
+        ename: String(e.ename ?? ''),
+        evalue: String(e.evalue ?? ''),
+        traceback: Array.isArray(e.traceback) ? (e.traceback as string[]) : [],
+      };
+    }
+  }
+  return null;
+}
+
+/**
+ * Build a CellContext from a CodeCell and its parent NotebookPanel.
+ * Returns null if essential data is missing.
+ *
+ * @param cell The code cell to inspect.
+ * @param panel The notebook panel that contains the cell.
+ */
+export function extractCellContext(
+  cell: CodeCell,
+  panel: NotebookPanel
+): CellContext | null {
+  if (!cell || !panel) {
+    return null;
+  }
+
+  const cells = panel.content.widgets;
+  const cellIndex = cells ? cells.indexOf(cell) : -1;
+  if (cellIndex < 0) {
+    return null;
+  }
+
+  const totalCells = cells ? cells.length : 0;
+  const previousCell = cellIndex > 0 ? cells[cellIndex - 1] : null;
+  const previousCode = previousCell
+    ? previousCell.model.sharedModel.getSource()
+    : null;
+
+  // Language: use 'python' as default for code cells.
+  // Future improvement: read from kernel info_reply.
+  const language = 'python';
+
+  let error: ErrorOutput | null = null;
+  if (cell.model.type === 'code') {
+    error = collectErrorOutputs(cell.model as ICodeCellModel);
+  }
+
+  return {
+    notebookPath: panel.context.path,
+    cellId: cell.model.id,
+    language,
+    cellIndex,
+    totalCells,
+    source: cell.model.sharedModel.getSource(),
+    previousCode,
+    error,
+  };
+}