refactor(reasoning): make channel plugins own reasoning rendering

Reasoning was being shipped to every channel as a generic progress
message with a `_reasoning: true` flag. Two problems with that:

1. Channels without a low-emphasis UI primitive (Telegram, Slack,
   Discord, Feishu...) would dump raw model thoughts as ordinary
   replies, polluting the conversation.
2. The agent loop double-gated by inspecting `channels_config`, which
   coupled the loop to display policy.

Treat reasoning as its own plugin action — `BaseChannel.send_reasoning`
defaults to a documented no-op; channels that have a fitting affordance
override. ChannelManager routes `_reasoning` outbounds to that method
only when the channel opts in via `show_reasoning` (camelCase alias
`showReasoning` mirrors `sendProgress`). Plugins that don't override
silently drop reasoning — "no fit, no leak" is the contract.

Reference implementation lands for WebSocket / WebUI: a new
`kind: "reasoning"` frame, parked on the active assistant bubble as a
collapsible `Thinking` group above the answer. CLI keeps its existing
direct path (it doesn't go through the bus). `ChannelsConfig.show_reasoning`
flips to `true` by default — only adapted channels surface anything,
others stay quiet.

Loop net diff is -3 lines: the `channels_config.show_reasoning` check
moves out, leaving emit_reasoning a one-liner that publishes and trusts
the channel to decide.

Co-authored-by: Cursor <cursoragent@cursor.com>

nanobot/channels/websocket.py

@@ -1487,6 +1487,30 @@ 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.
+        """
+        conns = list(self._subs.get(msg.chat_id, ()))
+        if not conns:
+            return
+        if not msg.content:
+            return
+        payload: dict[str, Any] = {
+            "event": "message",
+            "chat_id": msg.chat_id,
+            "text": msg.content,
+            "kind": "reasoning",
+        }
+        if msg.reply_to:
+            payload["reply_to"] = msg.reply_to
+        raw = json.dumps(payload, ensure_ascii=False)
+        for connection in conns:
+            await self._safe_send_to(connection, raw, label=" reasoning ")
+
     async def send_delta(
         self,
         chat_id: str,