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/channels/websocket.py

@@ -1487,30 +1487,54 @@ class WebSocketChannel(BaseChannel):
         for connection in conns:
             await self._safe_send_to(connection, raw, label=" ")
 
-    async def send_reasoning(self, msg: OutboundMessage) -> None:
-        """Stream model reasoning as a subordinate trace frame.
-
-        Renders as ``kind=reasoning`` alongside the existing ``tool_hint`` /
-        ``progress`` frames; the WebUI mounts these on the active assistant
-        bubble rather than as a conversational reply.
+    async def send_reasoning_delta(
+        self,
+        chat_id: str,
+        delta: str,
+        metadata: dict[str, Any] | None = None,
+    ) -> None:
+        """Push one chunk of model reasoning. Mirrors ``send_delta`` shape so
+        WebUI receives a stream that opens, updates in place, and closes —
+        rendered above the active assistant bubble with a shimmer header
+        until the matching ``reasoning_end`` arrives.
         """
-        conns = list(self._subs.get(msg.chat_id, ()))
-        if not conns:
+        conns = list(self._subs.get(chat_id, ()))
+        if not conns or not delta:
             return
-        if not msg.content:
-            return
-        payload: dict[str, Any] = {
-            "event": "message",
-            "chat_id": msg.chat_id,
-            "text": msg.content,
-            "kind": "reasoning",
+        meta = metadata or {}
+        body: dict[str, Any] = {
+            "event": "reasoning_delta",
+            "chat_id": chat_id,
+            "text": delta,
         }
-        if msg.reply_to:
-            payload["reply_to"] = msg.reply_to
-        raw = json.dumps(payload, ensure_ascii=False)
+        stream_id = meta.get("_stream_id")
+        if stream_id is not None:
+            body["stream_id"] = stream_id
+        raw = json.dumps(body, ensure_ascii=False)
         for connection in conns:
             await self._safe_send_to(connection, raw, label=" reasoning ")
 
+    async def send_reasoning_end(
+        self,
+        chat_id: str,
+        metadata: dict[str, Any] | None = None,
+    ) -> None:
+        """Close the current reasoning stream segment for in-place renderers."""
+        conns = list(self._subs.get(chat_id, ()))
+        if not conns:
+            return
+        meta = metadata or {}
+        body: dict[str, Any] = {
+            "event": "reasoning_end",
+            "chat_id": chat_id,
+        }
+        stream_id = meta.get("_stream_id")
+        if stream_id is not None:
+            body["stream_id"] = stream_id
+        raw = json.dumps(body, ensure_ascii=False)
+        for connection in conns:
+            await self._safe_send_to(connection, raw, label=" reasoning_end ")
+
     async def send_delta(
         self,
         chat_id: str,