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/i18n/locales/en/common.json

@@ -332,6 +332,7 @@
     "assistantTyping": "Assistant is typing",
     "toolSingle": "Using a tool",
     "toolMany": "Used {{count}} tools",
+    "reasoning": "Thinking",
     "imageAttachment": "Image attachment",
     "copyReply": "Copy reply",
     "copiedReply": "Copied reply"

webui/src/i18n/locales/zh-CN/common.json

@@ -320,6 +320,7 @@
     "assistantTyping": "助手正在输入",
     "toolSingle": "正在使用工具",
     "toolMany": "已使用 {{count}} 个工具",
+    "reasoning": "思考中",
     "imageAttachment": "图片附件",
     "copyReply": "复制回复",
     "copiedReply": "已复制回复"