Add pinned subagent status widget at bottom of UI

Serhioromano

### Request Pin an agent information widget to the bottom of the UI that shows the current process and a "Subagent started" message, so the user can visually understand what is going on at a glance. ### Use Case When a subagent spawns to handle a task (e.g., running `/ri`, executing diagnostics, or creating an issue), there is currently no persistent visual indicator. The user may not realize a subagent is active, leading to confusion about why things are happening or whether the agent is still working. ### Proposed Behavior - A small status bar/widget pinned at the bottom of the chat/UI - Displays the subagent's current activity/process name - Shows a clear message like "Subagent started" when a subagent kicks off - Updates or dismisses when the subagent completes ### Desired Outcome - Users always know when a subagent is running - Clear visual separation between main agent and subagent activity - No need to dig through logs to understand what's happening

Serhioromano

Implemented subagent status widget via `ctx.ui.setWidget`: - **On subagent start**: Sets a `belowEditor` widget showing ⏳ Subagent started — Creating issue on owner/repo... - **On success**: Updates to ✅ Subagent completed — <issue URL> - **On failure**: Updates to ❌ Subagent failed — <error message> - **Auto-dismiss**: Widget is cleared after 5 seconds via `setTimeout` Files changed: - `src/index.ts` — Added `ctx.ui.setWidget()` calls in the `/ri` command handler and promise callbacks - `CHANGELOG.md` — Added entry under [Unreleased] - `README.md` — Updated How It Works section to mention the status widget