Collapsible worker/background agent panel showing active sub-agents, model, reasoning level, and task description

### Describe the feature or problem you'd like to solve

When the main agent spawns background or worker sub-agents (e.g. via the `task` tool — `explore`, `task`, `general-purpose`, `code-review`), there is no at-a-glance visibility into what is running. You can't see how many sub-agents are active, which model they're using, what reasoning level they're at, or what they're working on — without waiting for them to finish and report back. For complex sessions with multiple parallel agents, this is a blind spot.

### Proposed solution

Add an optional **Worker Agents panel** in the TUI — collapsible/expandable — that shows a live list of all active background and worker sub-agents. The panel has two states:

**Folded (compact, default on):**
```
⚙ Workers [3 active]
```

**Unfolded (expanded):**
```
⚙ Workers [3 active]
● explore claude-haiku-4.5 — "Finding auth-related files in src/"
● task claude-haiku-4.5 — "Running test suite"
● general-purpose claude-sonnet-4.5 high — "Refactoring login handler"
```

Each row shows:
- **Status indicator** — spinning/active (●), done (✔), failed (✖)
- **Agent type** — `explore`, `task`, `general-purpose`, `code-review`
- **Model** — e.g. `claude-sonnet-4.5`, `claude-haiku-4.5`
- **Reasoning level** *(optional, shown only when non-default)* — e.g. `high`
- **Task description** *(optional, truncated)* — the short description passed to the agent

**Controls:**
- Toggle fold/unfold: dedicated keyboard shortcut (e.g. `Ctrl+W` or similar — subject to terminal keybinding availability) or by clicking the panel header
- Panel can be fully disabled via config (`"workerPanel": false` in `config.json`) for users who prefer a minimal UI
- Completed agents fade/clear from the panel after a short delay (configurable)

### Example prompts or workflows

1. Main agent launches 3 parallel `explore` sub-agents. User sees `⚙ Workers [3 active]` badge immediately, expands to see what each is scanning.
2. A `task` agent running tests fails. Panel shows `✖ task claude-haiku-4.5 — "Running test suite"` in red — user knows before the main agent reports back.
3. User prefers a clean UI — sets `"workerPanel": false` in `config.json`, panel never appears.
4. Long-running `general-purpose` agent with `high` reasoning — user can confirm the right reasoning level is being used without having to wait for the result.
5. User is on a metered plan and wants to monitor which models are being invoked in real time.

### Additional context

- OS: Windows 11 Pro (Build 26200 / 24H2)
- Shell: PowerShell 7.5.4
- Copilot CLI: 0.0.420 (win32-x64)
- The existing TUI already renders collapsible alt-screen panels (checkpoints, context, MCP) — this panel follows the same pattern.
- `config.json` already carries `reasoning_effort` — surfacing per-agent reasoning level in the panel is a natural extension.
- Model names shown in examples reflect the bundled agent defaults (`explore` and `task` use `claude-haiku-4.5`; `general-purpose` and `code-review` use `claude-sonnet-4.5`). The panel should reflect whatever model is actually configured or overridden at runtime.