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/tests/message-bubble.test.tsx

@@ -103,6 +103,39 @@ describe("MessageBubble", () => {
     expect(container.querySelector("video[controls]")).toBeInTheDocument();
   });
 
+  it("surfaces reasoning content above the assistant answer when provided", () => {
+    const message: UIMessage = {
+      id: "a-reasoning",
+      role: "assistant",
+      content: "The answer is 42.",
+      createdAt: Date.now(),
+      reasoning: ["Step 1: parse intent.", "Step 2: compute."],
+    };
+
+    render(<MessageBubble message={message} />);
+
+    expect(screen.getByText("Thinking")).toBeInTheDocument();
+    expect(screen.getByText(/Step 1: parse intent\./)).toBeInTheDocument();
+    expect(screen.getByText(/Step 2: compute\./)).toBeInTheDocument();
+    expect(screen.getByText("The answer is 42.")).toBeInTheDocument();
+  });
+
+  it("collapses the reasoning section when toggled", () => {
+    const message: UIMessage = {
+      id: "a-reasoning-collapse",
+      role: "assistant",
+      content: "done",
+      createdAt: Date.now(),
+      reasoning: ["hidden after toggle"],
+    };
+
+    render(<MessageBubble message={message} />);
+
+    expect(screen.getByText("hidden after toggle")).toBeInTheDocument();
+    fireEvent.click(screen.getByRole("button", { name: /thinking/i }));
+    expect(screen.queryByText("hidden after toggle")).not.toBeInTheDocument();
+  });
+
   it("renders assistant image media as a larger generated result", () => {
     const message: UIMessage = {
       id: "a-image",