feat: async + SSE message flow with interactive permission/question UI

Replace the synchronous /edit (wait for full markdown response) with an
async + SSE flow per demo.html so the user sees text stream in real-time
and can interact with permission/question events the agent raises.

Backend
-------
- EditHandler now calls OpenCode /session/:id/prompt_async and returns
  immediately with {ok, sessionId, notebookPath}; the LLM reply is no
  longer embedded in this response.
- New GlobalEventHandler proxies OpenCode /global/event as
  text/event-stream. Server forwards ALL events; the client filters.
  A too-eager server-side ?session= filter was silently dropping events
  the client would have accepted, so it was removed.
- New PermissionReplyHandler + QuestionReplyHandler forward user
  replies (once/always/reject and freeform answer) back to OpenCode
  Serve at /session/:sid/permissions/:permId and
  /session/:sid/question/:qId/reply.
- OpenCodeClient gains send_message_async(), stream_global_events()
  (async generator over the SSE feed via tornado streaming_callback),
  reply_permission() and reply_question(). Legacy send_message_sync
  removed.

Frontend
--------
- subscribeOpenCodeEvents() opens a fetch+reader SSE client (XSRF
  token injected from serverSettings); AbortController-backed close()
  is idempotent.
- OpenCodeInlinePrompt.applyEvent() routes events into the streaming
  UI: text delta -> assistant message (re-rendered as markdown on
  every delta so the user sees formatted <pre><code> blocks in
  real-time, not raw fence source); reasoning/tool/permission/question
  get collapsible details blocks. session.idle resets stream pointers
  and fires onStreamEnd.
- Permission/question blocks render real interactive UI: three
  buttons (once/always/reject) for permission, a text input + submit
  for question. Click handlers post through the new API routes and
  show success/failure status in place.
- Centralized idle detection (4 shapes: top-level + payload-nested
  x {session.idle, session.status/idle}) inside the prompt; cell
  action reacts via onStreamEnd callback rather than re-parsing
  event types.
- System / workspace / pty / lsp / mcp / installation events AND
  session-level control events (agent.switched, model.switched,
  file.edited) are not rendered in the frontend.

Tests
-----
- 52 backend pytest (was 43)
- 58 frontend jest (was 46)

design.md section 3 updated for the new async /edit response shape
and the new GET /events SSE endpoint.

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

design.md

@@ -62,7 +62,7 @@
 
 ## 🔌 3. 接口契约设计 (API Contract)
 
-### 接口：`POST /opencode-bridge/edit`（v2 — 无 `mode`）
+### 接口：`POST /opencode-bridge/edit`（v4 — 异步 + SSE）
 
 **Request Payload**（前端 `OpenCodeCellActions` 在用户提交 inline 输入框时发出）：
 
@@ -84,20 +84,40 @@
 }
 ```
 
-> **v2 变化**：body 不再含 `mode` / `action` 字段。前端只把 cell 的 input/output/error 收集到 `context`，加用户的自然语言 `prompt` 一起发出；后端用统一的 `UNIFIED_SYSTEM_PROMPT` + LLM 自己判断优化/排错/编辑。`providerId` / `modelId` 来自 JupyterLab settings 透传。
+> body 不含 `mode` / `action`。`providerId` / `modelId` 来自 inline 输入框动态选择，无 settings 持久化。
 
-**Response**（当前 v2 实现：单次 JSON；SSE 流式属于 Slice 4 后续工作）：
+**Response**（v4 异步）：后端调用 OpenCode `/session/:id/prompt_async` 后**立即返回**——不等 LLM 完成。
 
 ```json
 {
   "ok": true,
-  "finalSource": "import plotly.express as px\nfig = px.line(x=[1, 2, 3], y=[1, 2, 3])\nfig.show()",
   "sessionId": "ses-abc123",
   "notebookPath": "analysis/demo.ipynb"
 }
 ```
 
-> 成功时前端用 `finalSource` 就地替换 cell 源码（`cell.model.sharedModel.setSource`）。Session 由服务端 `SessionManager` 按 `notebookPath` 复用（1 notebook 1 sid），404 / `session not found` 时服务端自动 invalidate 并在下次请求重建。
+> 响应中**无** `markdown` 字段。AI 的回复通过独立的 SSE 端点（`/opencode-bridge/events`）实时推送给前端，前端在 inline prompt 的 history 区里增量渲染文本流 / 推理 / 工具调用 / 权限提问等。Session 仍由 `SessionManager` 按 `notebookPath` 复用（1 notebook 1 sid），404 / `session not found` 时服务端自动 invalidate 并在下次请求重建。cell 源码**不**被替换；用户可对 AI 消息中的每个代码块使用 Copy / Insert / Replace 按钮。
+
+### 接口：`GET /opencode-bridge/events?session=<sid>`（v4 新增 — SSE 代理）
+
+服务端长连接到 OpenCode Serve 的 `GET /global/event`，按 `session=<sid>` 过滤后以 `text/event-stream` 透传给前端。OpenCode 事件 schema 较松散，事件示例：
+
+```json
+{"type": "session.next.text.delta",        "properties": {"sessionID": "ses-abc123", "delta": "hel"}}
+{"type": "session.next.text.delta",        "properties": {"sessionID": "ses-abc123", "delta": "lo"}}
+{"type": "session.next.reasoning.delta",   "properties": {"sessionID": "ses-abc123", "delta": "thinking..."}}
+{"type": "session.next.tool.called",       "properties": {"sessionID": "ses-abc123", "name": "bash"}}
+{"type": "session.next.tool.input.delta",  "properties": {"sessionID": "ses-abc123", "delta": "{\"cmd\": \"ls\"}"}}
+{"type": "session.next.tool.success",      "properties": {"sessionID": "ses-abc123", "result": {"exitCode": 0}}}
+{"type": "session.next.agent.switched",    "properties": {"sessionID": "ses-abc123", "agentID": "build"}}
+{"type": "session.next.model.switched",    "properties": {"sessionID": "ses-abc123", "modelID": "claude-..."}}
+{"type": "file.edited",                    "properties": {"sessionID": "ses-abc123", "path": "foo.py"}}
+{"type": "permission.asked",               "properties": {"sessionID": "ses-abc123", "id": "perm-1", "description": "rm -rf"}}
+{"type": "question.asked",                 "properties": {"sessionID": "ses-abc123", "id": "q-1", "question": "Which env?"}}
+{"type": "session.idle",                   "properties": {"sessionID": "ses-abc123"}}
+```
+
+> v4 前端 `OpenCodeInlinePrompt.applyEvent()` 按 `type` 路由到不同渲染器：text/reasoning/tool 块用 `<details>` 可折叠，permission/question 提示独立块（响应需要 OpenCode Serve 提供对应 API；本扩展 v4 仅渲染，不实现 allow/answer 回调路由）。`session.idle` 触发流指针重置、关闭 SSE 订阅、重新启用输入框。System 事件（`server.*` / `workspace.*` / `lsp.*` / `mcp.*` / `pty.*` / `installation.*`）以日志行形式展示，不参与 session 过滤。
 
 ---