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_cell_actions.spec.ts

@@ -47,6 +47,11 @@ jest.mock('@lumino/widgets', () => {
   return { Widget };
 });
 
+jest.mock('marked', () => ({
+  __esModule: true,
+  marked: { parse: (md: string) => `<mock>${md}</mock>` }
+}));
+
 import {
   OpenCodeCellActions,
   setOpenCodeProviders,
@@ -324,6 +329,33 @@ describe('OpenCodeCellActions', () => {
       cell.node.querySelector('.opencode-inline-prompt .opencode-model-select')
     ).toBeNull();
   });
+
+  it('on a successful response, does NOT replace the cell source (renders to inline output instead)', () => {
+    setOpenCodeProviders(null);
+    const cell = makeFakeCell('x = 1');
+    const actions = new OpenCodeCellActions(cell);
+    // Click once to create the inline prompt (so setOutput has a target).
+    Object.defineProperty(actions, 'parent', { value: cell, configurable: true });
+    (actions as any).onAfterAttach({} as any);
+    const btn = actions.node.querySelector('button') as HTMLButtonElement;
+    btn.click();
+
+    // Simulate a successful response.
+    (actions as any)._handleResponse({
+      ok: true,
+      markdown: '# AI says hi\n\n```python\nprint("hi")\n```',
+      sessionId: 'ses-abc',
+      notebookPath: 'foo.ipynb'
+    });
+
+    // The cell source must remain unchanged.
+    expect((cell as any).model.sharedModel.setSource).not.toHaveBeenCalled();
+    // The inline output area is now visible with the rendered markdown.
+    const output = cell.node.querySelector(
+      '.opencode-inline-prompt .opencode-inline-output'
+    ) as HTMLElement;
+    expect(output.style.display).toBe('block');
+  });
 });
 
 describe('OpenCodeInlinePrompt', () => {
@@ -336,6 +368,34 @@ describe('OpenCodeInlinePrompt', () => {
       providers: null
     });
     expect(prompt.node.querySelector('textarea')).not.toBeNull();
-    expect(prompt.node.querySelectorAll('button').length).toBe(2);
+    // 3 buttons: 发送, 取消, and the output area close (✕).
+    expect(prompt.node.querySelectorAll('button').length).toBe(3);
+  });
+
+  it('hides the output area by default; setOutput reveals it with rendered markdown', () => {
+    const cell = makeFakeCell('x = 1');
+    const prompt = new OpenCodeInlinePrompt(cell, {
+      onSubmit: jest.fn(),
+      onCancel: jest.fn(),
+      disabled: false,
+      providers: null
+    });
+    const output = prompt.node.querySelector(
+      '.opencode-inline-prompt .opencode-inline-output'
+    ) as HTMLElement;
+    expect(output).not.toBeNull();
+    expect(output.style.display).toBe('none');
+
+    prompt.setOutput('# Hello\n\n```python\nx = 1\n```');
+    expect(output.style.display).toBe('block');
+    const content = prompt.node.querySelector(
+      '.opencode-inline-prompt .opencode-inline-output-content'
+    ) as HTMLElement;
+    // marked is mocked to wrap the input in <mock>...</mock>.
+    expect(content.innerHTML).toContain('<mock>');
+    expect(content.innerHTML).toContain('# Hello');
+
+    prompt.hideOutput();
+    expect(output.style.display).toBe('none');
   });
 });