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>

tests/channels/test_websocket_channel.py

@@ -358,6 +358,60 @@ async def test_send_delta_emits_delta_and_stream_end() -> None:
     assert second["stream_id"] == "sid"
 
 
+@pytest.mark.asyncio
+async def test_send_reasoning_emits_reasoning_kind_frame() -> None:
+    bus = MagicMock()
+    channel = WebSocketChannel({"enabled": True, "allowFrom": ["*"]}, bus)
+    mock_ws = AsyncMock()
+    channel._attach(mock_ws, "chat-1")
+
+    await channel.send_reasoning(OutboundMessage(
+        channel="websocket",
+        chat_id="chat-1",
+        content="step-by-step thinking",
+        metadata={"_progress": True, "_reasoning": True},
+    ))
+
+    mock_ws.send.assert_awaited_once()
+    payload = json.loads(mock_ws.send.await_args.args[0])
+    assert payload["event"] == "message"
+    assert payload["chat_id"] == "chat-1"
+    assert payload["text"] == "step-by-step thinking"
+    assert payload["kind"] == "reasoning"
+
+
+@pytest.mark.asyncio
+async def test_send_reasoning_drops_empty_content() -> None:
+    """Empty reasoning emits nothing — keeps the frontend bubble clean."""
+    bus = MagicMock()
+    channel = WebSocketChannel({"enabled": True, "allowFrom": ["*"]}, bus)
+    mock_ws = AsyncMock()
+    channel._attach(mock_ws, "chat-1")
+
+    await channel.send_reasoning(OutboundMessage(
+        channel="websocket",
+        chat_id="chat-1",
+        content="",
+        metadata={"_reasoning": True},
+    ))
+
+    mock_ws.send.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_send_reasoning_without_subscribers_is_noop() -> None:
+    bus = MagicMock()
+    channel = WebSocketChannel({"enabled": True, "allowFrom": ["*"]}, bus)
+
+    await channel.send_reasoning(OutboundMessage(
+        channel="websocket",
+        chat_id="unattached",
+        content="thinking",
+        metadata={"_reasoning": True},
+    ))
+    # No subscribers, no exception, no send.
+
+
 @pytest.mark.asyncio
 async def test_send_turn_end_emits_turn_end_event() -> None:
     bus = MagicMock()