feat(reasoning): stream reasoning content as a first-class channel

Reasoning now flows as its own stream — symmetric to the answer's
``delta`` / ``stream_end`` pair — instead of being shipped as one
oversized progress message. This lets WebUI render a live "Thinking…"
bubble that updates in place, then auto-collapses when the stream
closes. Other channels remain plugin no-ops by default.

## Protocol

New metadata: ``_reasoning_delta`` (chunk) and ``_reasoning_end``
(close marker). ChannelManager routes both to the dedicated plugin
hooks below; the legacy one-shot ``_reasoning`` is kept for back-compat
and BaseChannel expands it into a single delta + end pair so plugins
only ever implement the streaming primitives.

WebSocket emits two new events:

- ``reasoning_delta`` (event, chat_id, text, optional stream_id)
- ``reasoning_end`` (event, chat_id, optional stream_id)

## BaseChannel surface

- ``send_reasoning_delta(chat_id, delta, metadata)`` — no-op default
- ``send_reasoning_end(chat_id, metadata)`` — no-op default
- ``send_reasoning(msg)`` — back-compat wrapper, base impl forwards
  to the streaming primitives

A channel adds reasoning support by overriding the two streaming
primitives. Telegram / Slack / Discord / Feishu / WeChat / Matrix keep
the base no-ops until their bubble UIs are adapted; reasoning silently
drops at dispatch, never as a stray text message.

## AgentHook

Adds ``emit_reasoning_end`` to the hook lifecycle. ``_LoopHook`` tracks
whether a reasoning segment is open and closes it on:

- the first answer delta arriving (so the UI locks the bubble before
  the answer renders below),
- ``on_stream_end``,
- one-shot ``reasoning_content`` / ``thinking_blocks`` after a single
  non-streaming response.

## WebUI

- ``UIMessage.reasoning`` is now a single accumulated string with a
  companion ``reasoningStreaming`` flag.
- ``useNanobotStream`` consumes ``reasoning_delta`` / ``reasoning_end``;
  legacy ``kind: "reasoning"`` is auto-translated to a delta + end.
- New ``ReasoningBubble``: shimmer header + auto-expanded while
  streaming, collapses to a clickable "Thinking" pill once closed,
  respects ``prefers-reduced-motion``.
- Answer deltas adopt the reasoning placeholder so the bubble and the
  answer share one assistant row.

## Tests

- ``tests/channels/test_channel_manager_reasoning.py`` — manager routes
  delta + end, drops on channel opt-out, expands one-shot back-compat.
- ``tests/channels/test_websocket_channel.py`` — new ``reasoning_delta``
  / ``reasoning_end`` frames, empty-chunk safety, no-subscriber safety,
  back-compat expansion.
- ``tests/agent/test_runner_reasoning.py`` — runner closes the segment
  on streaming answer start and after one-shot reasoning.
- WebUI ``useNanobotStream`` + ``message-bubble`` cover the new
  protocol and the shimmer styling.

## Docs

``docs/configuration.md`` and ``docs/websocket.md`` document the new
events and the plugin contract.

Co-authored-by: Cursor <cursoragent@cursor.com>

nanobot/agent/runner.py

@@ -291,6 +291,7 @@ class AgentRunner:
             response.content = cleaned_content
             if reasoning_text and not context.streamed_reasoning:
                 await hook.emit_reasoning(reasoning_text)
+                await hook.emit_reasoning_end()
                 context.streamed_reasoning = True
 
             if response.should_execute_tools:
@@ -617,6 +618,8 @@ class AgentRunner:
             and getattr(self.provider, "supports_progress_deltas", False) is True
         )
 
+        progress_state: dict[str, bool] | None = None
+
         if wants_streaming:
             async def _stream(delta: str) -> None:
                 if delta:
@@ -630,6 +633,7 @@ class AgentRunner:
         elif wants_progress_streaming:
             stream_buf = ""
             think_extractor = IncrementalThinkExtractor()
+            progress_state = {"reasoning_open": False}
 
             async def _stream_progress(delta: str) -> None:
                 nonlocal stream_buf
@@ -642,8 +646,12 @@ class AgentRunner:
 
                 if await think_extractor.feed(stream_buf, hook.emit_reasoning):
                     context.streamed_reasoning = True
+                    progress_state["reasoning_open"] = True
 
                 if incremental:
+                    if progress_state["reasoning_open"]:
+                        await hook.emit_reasoning_end()
+                        progress_state["reasoning_open"] = False
                     context.streamed_content = True
                     await spec.progress_callback(incremental)
 
@@ -654,16 +662,20 @@ class AgentRunner:
         else:
             coro = self.provider.chat_with_retry(**kwargs)
 
-        if timeout_s is None:
-            return await coro
         try:
-            return await asyncio.wait_for(coro, timeout=timeout_s)
+            response = (
+                await coro if timeout_s is None
+                else await asyncio.wait_for(coro, timeout=timeout_s)
+            )
         except asyncio.TimeoutError:
             return LLMResponse(
                 content=f"Error calling LLM: timed out after {timeout_s:g}s",
                 finish_reason="error",
                 error_kind="timeout",
             )
+        if progress_state and progress_state.get("reasoning_open"):
+            await hook.emit_reasoning_end()
+        return response
 
     async def _request_finalization_retry(
         self,