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/components/opencode_inline_prompt.ts

@@ -9,6 +9,7 @@
  */
 import { CodeCell } from '@jupyterlab/cells';
 import { Widget } from '@lumino/widgets';
+import { marked } from 'marked';
 
 import type { OpenCodeProvidersResponse } from '../types';
 
@@ -69,6 +70,8 @@ export class OpenCodeInlinePrompt extends Widget {
   private _modelSelect: HTMLSelectElement | null = null;
   private _providerModels: FlatProvider[];
   private _defaultMap: { [pid: string]: string };
+  private _outputArea: HTMLDivElement;
+  private _outputContent: HTMLDivElement;
 
   constructor(
     _cell: CodeCell,
@@ -153,6 +156,31 @@ export class OpenCodeInlinePrompt extends Widget {
     }
     this.node.appendChild(this._textarea);
     this.node.appendChild(actions);
+
+    // Output area: hidden by default; revealed by setOutput() with the
+    // markdown-rendered AI response. The cell source is NOT replaced.
+    this._outputArea = document.createElement('div');
+    this._outputArea.className = 'opencode-inline-output';
+    this._outputArea.style.display = 'none';
+    const outputHeader = document.createElement('div');
+    outputHeader.className = 'opencode-inline-output-header';
+    const outputTitle = document.createElement('span');
+    outputTitle.textContent = '结果';
+    const outputCloseBtn = document.createElement('button');
+    outputCloseBtn.className = 'opencode-inline-output-close';
+    outputCloseBtn.type = 'button';
+    outputCloseBtn.textContent = '✕';
+    outputCloseBtn.title = '关闭输出';
+    outputCloseBtn.addEventListener('click', () => {
+      this.hideOutput();
+    });
+    outputHeader.appendChild(outputTitle);
+    outputHeader.appendChild(outputCloseBtn);
+    this._outputContent = document.createElement('div');
+    this._outputContent.className = 'opencode-inline-output-content';
+    this._outputArea.appendChild(outputHeader);
+    this._outputArea.appendChild(this._outputContent);
+    this.node.appendChild(this._outputArea);
   }
 
   private _rebuildModelSelect(providerId: string | undefined): void {
@@ -196,4 +224,20 @@ export class OpenCodeInlinePrompt extends Widget {
       this._modelSelect.disabled = disabled;
     }
   }
+
+  /**
+   * Render the AI response as markdown and show the output area in place
+   * of replacing the cell source. Safe for local-trusted content from the
+   * user's OpenCode Serve; do not feed untrusted markdown here without
+   * adding a sanitizer (e.g. DOMPurify).
+   */
+  setOutput(markdown: string): void {
+    this._outputContent.innerHTML = marked.parse(markdown) as string;
+    this._outputArea.style.display = 'block';
+  }
+
+  hideOutput(): void {
+    this._outputArea.style.display = 'none';
+    this._outputContent.textContent = '';
+  }
 }