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/base.py

@@ -121,18 +121,53 @@ class BaseChannel(ABC):
         """
         pass
 
-    async def send_reasoning(self, msg: OutboundMessage) -> None:
-        """Surface model reasoning/thinking content.
+    async def send_reasoning_delta(
+        self, chat_id: str, delta: str, metadata: dict[str, Any] | None = None
+    ) -> None:
+        """Stream a chunk of model reasoning/thinking content.
 
         Default is no-op. Channels with a native low-emphasis primitive
         (Slack context block, Telegram expandable blockquote, Discord
         subtext, WebUI italic bubble, ...) override to render reasoning
-        as a subordinate trace. Channels without a suitable affordance
-        keep this no-op: silently dropping is better than leaking raw
-        model thoughts as regular conversational messages.
+        as a subordinate trace that updates in place as the model thinks.
+
+        Streaming contract mirrors :meth:`send_delta`: ``_reasoning_delta``
+        is a chunk, ``_reasoning_end`` ends the current reasoning segment,
+        and stateful implementations should key buffers by ``_stream_id``
+        rather than only by ``chat_id``.
         """
         return
 
+    async def send_reasoning_end(
+        self, chat_id: str, metadata: dict[str, Any] | None = None
+    ) -> None:
+        """Mark the end of a reasoning stream segment.
+
+        Default is no-op. Channels that buffer ``send_reasoning_delta``
+        chunks for in-place updates use this signal to flush and freeze
+        the rendered group; one-shot channels can ignore it entirely.
+        """
+        return
+
+    async def send_reasoning(self, msg: OutboundMessage) -> None:
+        """Deliver a complete reasoning block.
+
+        Default implementation reuses the streaming pair so plugins only
+        need to override the delta/end methods. Equivalent to one delta
+        with the full content followed immediately by an end marker —
+        keeps a single rendering path for both streamed and one-shot
+        reasoning (e.g. DeepSeek-R1's final-response ``reasoning_content``).
+        """
+        if not msg.content:
+            return
+        meta = dict(msg.metadata or {})
+        meta.setdefault("_reasoning_delta", True)
+        await self.send_reasoning_delta(msg.chat_id, msg.content, meta)
+        end_meta = dict(meta)
+        end_meta.pop("_reasoning_delta", None)
+        end_meta["_reasoning_end"] = True
+        await self.send_reasoning_end(msg.chat_id, end_meta)
+
     @property
     def supports_streaming(self) -> bool:
         """True when config enables streaming AND this subclass implements send_delta."""