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/base.py

@@ -28,6 +28,7 @@ class BaseChannel(ABC):
     transcription_language: str | None = None
     send_progress: bool = True
     send_tool_hints: bool = False
+    show_reasoning: bool = True
 
     def __init__(self, config: Any, bus: MessageBus):
         """
@@ -120,6 +121,18 @@ class BaseChannel(ABC):
         """
         pass
 
+    async def send_reasoning(self, msg: OutboundMessage) -> None:
+        """Surface 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.
+        """
+        return
+
     @property
     def supports_streaming(self) -> bool:
         """True when config enables streaming AND this subclass implements send_delta."""