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>

webui/src/globals.css

@@ -117,6 +117,34 @@
     --cjk-line-height: 1.625;
   }
 
+  /* Shimmer band sweeping across the reasoning header while
+     ``reasoning_delta`` frames are arriving. Pure CSS, no JS animation,
+     respects ``prefers-reduced-motion``. */
+  @keyframes reasoning-shimmer-sweep {
+    0% {
+      background-position: -200% 0;
+    }
+    100% {
+      background-position: 200% 0;
+    }
+  }
+  .reasoning-shimmer {
+    background-image: linear-gradient(
+      90deg,
+      transparent 0%,
+      hsl(var(--muted-foreground) / 0.18) 50%,
+      transparent 100%
+    );
+    background-size: 200% 100%;
+    background-repeat: no-repeat;
+    animation: reasoning-shimmer-sweep 2.2s linear infinite;
+  }
+  @media (prefers-reduced-motion: reduce) {
+    .reasoning-shimmer {
+      animation: none;
+    }
+  }
+
   /* Subtle scrollbar that doesn't fight the dark background. */
   .scrollbar-thin {
     scrollbar-width: thin;