Collapsible real-time session stats panel (tokens, context, API calls, premium requests, cache, duration)

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

There is currently no way to monitor session resource usage at a glance during an active session. To understand how much context has been consumed, how many API calls have been made, or how many premium requests have been used, a user must either wait for a crash/compaction or dig through logs after the fact. For power users, teams on metered plans, and anyone running long or expensive sessions, this lack of real-time visibility is a significant gap.

### Proposed solution

Add an optional **Session Stats panel** — collapsible/expandable, togglable via config — that renders a live compact summary of resource and performance metrics for the current session.

**Folded (compact, one-line):**
```
📊 Stats ctx 68% ↑42k/↓18k tok 🕒 12m 34s calls 37 premium 8
```

**Unfolded (expanded):**
```
📊 Session Stats
Context 68% used (87,040 / 128,000 tokens)
Tokens sent 42,310
Tokens received 18,045
Cached tokens 11,200 (21% of sent)
API calls 37 total (28 standard · 8 premium · 1 cached)
Premium requests 8
Duration 12m 34s
Avg. turn time ~20s
```

**Suggested stats to surface:**

| Stat | Description |
|---|---|
| Context usage | Current fill % and absolute tokens vs. window size |
| Tokens sent | Cumulative input tokens for the session |
| Tokens received | Cumulative output tokens for the session |
| Cached tokens | Tokens served from prompt cache (where applicable) |
| API calls | Total LLM requests, broken down by standard / premium / cached |
| Premium requests | Count of premium-model calls against plan quota |
| Session duration | Wall-clock time since session start |
| Avg. turn time | Average time per agent turn |

**Controls:**
- Toggle fold/unfold: a dedicated keyboard shortcut (the specific key should avoid conflicts — e.g. `Ctrl+S` sends XOFF in some terminals and should not be used; a safe alternative such as a function key or a free modifier combo should be chosen) or by clicking the panel header
- Opt-in via `config.json`:
```json
{ "statsPanel": true }
```
- Fully disabled by default to preserve a clean UI for new users; power users explicitly enable it.
- Stats persist and accumulate across compaction events so the total is always session-wide, not just the current context window.

### Example prompts or workflows

1. User on a metered Copilot plan monitors premium request count in real time — knows at a glance when they're approaching their quota before the session ends.
2. Long refactor session running for 45 minutes: user glances at the folded one-liner to see context is at 72% and decides to manually `/compact` before it auto-compacts.
3. User notices cached token % is low and adjusts their workflow to reuse more context, reducing cost.
4. Team lead shares a screenshot of the expanded stats panel with their team to communicate session cost and complexity.
5. User keeps the panel folded for minimal UI footprint — the single-line summary gives just enough signal without cluttering the screen.

### Additional context

- OS: Windows 11 Pro (Build 26200 / 24H2)
- Shell: PowerShell 7.5.4
- Copilot CLI: 0.0.420 (win32-x64)
- The `/context` command already surfaces context usage — this panel extends that concept to a persistent, live, multi-metric view accessible without leaving the main session flow.
- The collapsible panel pattern is already established in the TUI (checkpoints, MCP, context) — this fits the same model.
- Stats should survive context compaction and reflect true session totals, not just the current window's figures.