feat: inline markdown output box — render AI reply with marked, do not replace cell

Flow (v3-final corrected):
  1. Model call succeeds.
  2. Server passes the AI reply through unchanged as 'markdown' (the
     system prompt allows ```language fences + a brief explanation, so
     the response is real markdown that marked can render into code
     blocks, headings, etc.).
  3. Client OpenCodeCellActions._handleResponse calls prompt.setOutput(
     resp.markdown); the inline prompt widget renders it with
     marked.parse and shows it in a new output area (hidden until a
     response arrives). The cell source is NOT replaced.
  4. User can close the output or cancel the whole prompt.

Server:
  - New unified system prompt: '你是代码助手 ... 按指令修改代码，可附简
    短说明' (allows ```fences``` + explanation; no more 'no markdown
    fences' restriction).
  - EditHandler returns {ok, markdown, sessionId, notebookPath} (raw
    text, fences intact). _strip_code_fence kept as a helper for any
    future apply-to-cell path; no longer called.
  - finalSource field dropped (the cell-apply path is gone).

Client:
  - OpenCodeSuccess: markdown: string (finalSource removed).
  - OpenCodeInlinePrompt: new output area, setOutput(md) renders via
    marked.parse, hideOutput() closes it.
  - OpenCodeCellActions._handleResponse: setOutput(markdown) instead of
    sharedModel.setSource + auto-hide. The prompt stays open so the
    user can read the output.
  - Uses marked@17 (already in node_modules via JupyterLab; no new dep).
  - CSS: output area styling (border, max-height 320px scroll, code/pre
    styling, close button).

Tests: pytest 34/34, jest 29/29. marked is mocked in jest.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/__tests__/opencode_client.spec.ts

@@ -85,7 +85,7 @@ describe('callOpenCodeEdit', () => {
   it('POSTs to /opencode-bridge/edit with JSON body', async () => {
     const respBody = {
       ok: true,
-      finalSource: 'def foo(x: int) -> int: return x',
+      markdown: '```python\ndef foo(x: int) -> int: return x\n```',
       sessionId: 'sid',
       notebookPath: 'foo.ipynb',
     };
@@ -107,7 +107,7 @@ describe('callOpenCodeEdit', () => {
     );
     expect(resp.ok).toBe(true);
     if (resp.ok) {
-      expect(resp.finalSource).toContain('int');
+      expect(resp.markdown).toContain('int');
     }
   });