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/manager.py

@@ -283,13 +283,18 @@ class ChannelManager:
                         timeout=1.0
                     )
 
-                if msg.metadata.get("_reasoning"):
-                    # Reasoning rides its own plugin channel: only delivered when
-                    # the destination channel both opts in (``show_reasoning``)
-                    # and overrides ``send_reasoning``. Channels without a
-                    # low-emphasis UI primitive keep the base no-op and the
-                    # content silently drops here rather than leak as a
-                    # conversational reply.
+                if (
+                    msg.metadata.get("_reasoning_delta")
+                    or msg.metadata.get("_reasoning_end")
+                    or msg.metadata.get("_reasoning")
+                ):
+                    # Reasoning rides its own plugin channel: only delivered
+                    # when the destination channel opts in via ``show_reasoning``
+                    # and overrides the streaming primitives. Channels without
+                    # a low-emphasis UI affordance keep the base no-op and the
+                    # content silently drops here. ``_reasoning`` (one-shot)
+                    # is accepted for backward compatibility with hooks that
+                    # haven't migrated to delta/end yet.
                     channel = self.channels.get(msg.channel)
                     if channel is not None and channel.show_reasoning:
                         await self._send_with_retry(channel, msg)
@@ -345,7 +350,14 @@ class ChannelManager:
     @staticmethod
     async def _send_once(channel: BaseChannel, msg: OutboundMessage) -> None:
         """Send one outbound message without retry policy."""
-        if msg.metadata.get("_reasoning"):
+        if msg.metadata.get("_reasoning_end"):
+            await channel.send_reasoning_end(msg.chat_id, msg.metadata)
+        elif msg.metadata.get("_reasoning_delta"):
+            await channel.send_reasoning_delta(msg.chat_id, msg.content, msg.metadata)
+        elif msg.metadata.get("_reasoning"):
+            # Back-compat: one-shot reasoning. BaseChannel translates this
+            # to a single delta + end pair so plugins only implement the
+            # streaming primitives.
             await channel.send_reasoning(msg)
         elif msg.metadata.get("_stream_delta") or msg.metadata.get("_stream_end"):
             await channel.send_delta(msg.chat_id, msg.content, msg.metadata)