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>

tests/agent/test_runner_reasoning.py

@@ -24,11 +24,15 @@ class _RecordingHook(AgentHook):
     def __init__(self) -> None:
         super().__init__()
         self.emitted: list[str] = []
+        self.end_calls = 0
 
     async def emit_reasoning(self, reasoning_content: str | None) -> None:
         if reasoning_content:
             self.emitted.append(reasoning_content)
 
+    async def emit_reasoning_end(self) -> None:
+        self.end_calls += 1
+
 
 @pytest.mark.asyncio
 async def test_runner_preserves_reasoning_fields_in_assistant_history():
@@ -277,3 +281,41 @@ async def test_runner_does_not_double_emit_when_inline_think_already_streamed():
 
     assert result.final_content == "The answer."
     assert hook.emitted == ["working..."]
+    assert hook.end_calls >= 1, "reasoning stream must be closed once the answer starts"
+
+
+@pytest.mark.asyncio
+async def test_runner_closes_reasoning_stream_after_one_shot_response():
+    """A non-streaming response carrying ``reasoning_content`` must emit
+    both a reasoning delta and an end marker so channels can finalize the
+    in-place bubble."""
+    from nanobot.agent.runner import AgentRunSpec, AgentRunner
+
+    provider = MagicMock()
+
+    async def chat_with_retry(**kwargs):
+        return LLMResponse(
+            content="answer",
+            reasoning_content="hidden thought",
+            tool_calls=[],
+            usage={"prompt_tokens": 5, "completion_tokens": 3},
+        )
+
+    provider.chat_with_retry = chat_with_retry
+    tools = MagicMock()
+    tools.get_definitions.return_value = []
+
+    hook = _RecordingHook()
+    runner = AgentRunner(provider)
+    result = await runner.run(AgentRunSpec(
+        initial_messages=[{"role": "user", "content": "q"}],
+        tools=tools,
+        model="test-model",
+        max_iterations=3,
+        max_tool_result_chars=_MAX_TOOL_RESULT_CHARS,
+        hook=hook,
+    ))
+
+    assert result.final_content == "answer"
+    assert hook.emitted == ["hidden thought"]
+    assert hook.end_calls == 1