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>

webui/src/lib/types.ts

@@ -44,6 +44,10 @@ export interface UIMessage {
   images?: UIImage[];
   /** Signed or local UI-renderable media attachments. */
   media?: UIMediaAttachment[];
+  /** Assistant turn: model reasoning / thinking content collected from
+   * `kind: "reasoning"` frames. Each entry is one emit cycle, joined with
+   * blank lines on render. */
+  reasoning?: string[];
 }
 
 export interface ChatSummary {
@@ -141,7 +145,7 @@ export type InboundEvent =
       media_urls?: Array<{ url: string; name?: string }>;
       /** Present when the frame is an agent breadcrumb (e.g. tool hint,
        * generic progress line) rather than a conversational reply. */
-      kind?: "tool_hint" | "progress";
+      kind?: "tool_hint" | "progress" | "reasoning";
     }
   | {
       event: "delta";