Global session history registry and copilot --history flag — persistent cross-session stats and audit log accessible without starting the agent

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

Every Copilot CLI session is isolated — when it ends, its context and metadata are effectively gone from any accessible view. There is no persistent, cross-session record of what was run, where, when, for how long, with which model, or at what cost. Users who want to understand their usage patterns, audit past sessions, track productivity, or simply recall what they worked on last Tuesday have no facility to do so. The only existing data is buried in per-session `events.jsonl` files with no aggregated view or queryable interface.

Additionally, this information should be accessible **without launching the full agent** — querying session history should not consume a premium request or spin up the LLM.

### Proposed solution

Introduce a **persistent global session registry** — a lightweight, append-only index written to `~/.copilot/session-history.db` — that records a structured summary for every session at open and close time. This data is then exposed via:

1. **A `/history` slash command** — inside an active session, for interactive browsing
2. **A `copilot --history` CLI flag** — invokable directly from the shell, **without starting the agent or consuming any quota**

**Data captured per session:**

| Field | Description |
|---|---|
| Session ID | Unique session identifier |
| Started at | Timestamp of session start |
| Ended at | Timestamp of clean exit (or `crashed` / `killed`) |
| Duration | Wall-clock duration |
| Working directory | Absolute path at session start |
| Git branch | Branch name at start (if in a repo) |
| Model | Primary model used |
| Reasoning effort | `low` / `medium` / `high` |
| Total turns | Number of prompts submitted |
| Tokens sent | Cumulative input tokens |
| Tokens received | Cumulative output tokens |
| Cached tokens | Tokens served from prompt cache |
| API calls | Total LLM requests |
| Premium requests | Count of premium-model calls |
| MCP servers | Names of active MCP servers |
| Sub-agents launched | Count and types |
| Compactions | Number of context compaction events |
| Exit status | `clean` / `crashed` / `killed` / `timed-out` |

**`copilot --history` output (shell, no agent, no quota):**

```
$ copilot --history

GitHub Copilot CLI — Session History (last 10 sessions)

# Started Duration Dir Model Turns Tokens↑ Premium Exit
─────────────────────────────────────────────────────────────────────────────────────────────────────────────
1 2026-03-03 01:05 48m 12s ~/repos/myapp claude-sonnet-4.5 14 118,420 6 clean
2 2026-03-02 22:14 12m 03s ~/repos/myapp claude-sonnet-4.5 5 31,200 2 clean
3 2026-03-02 18:40 3m 55s ~/repos/timer claude-sonnet-4.5 2 8,100 1 crashed ⚠
4 2026-03-01 15:22 1h 02m ~/repos/myapp claude-opus-4.5 21 290,000 21 clean
...

Totals (all time): 48 sessions · 18h 32m · 1.2M tokens sent · 89 premium requests
```

**Flags:**
```
copilot --history # Show last 10 sessions (default)
copilot --history --all # Show all sessions
copilot --history --limit 25 # Show last N sessions
copilot --history --json # Machine-readable JSON output
copilot --history --stats # Aggregate totals only (no per-session rows)
copilot --history --dir . # Filter to sessions started in current directory
copilot --history --since 7d # Sessions from last 7 days
```

**`/history` inside the agent** opens the same data in an interactive alt-screen table with sorting, filtering, and the ability to select a past session row and ask the agent questions about it.

### Example prompts or workflows

1. `copilot --history --stats` — user checks monthly premium request consumption before their billing cycle resets, zero tokens spent.
2. `copilot --history --dir . --since 30d` — user reviews all sessions in the current repo over the last month to recap progress for a sprint review.
3. `/history` inside the agent, user selects a past session row and asks "summarise what I did" — agent uses the session metadata as context.
4. `copilot --history --json | jq` — power user pipes session data into a custom dashboard or spreadsheet.
5. DevOps team runs `copilot --history --json` in a post-session hook to log usage telemetry to an internal system.
6. User sees `crashed ⚠` in the exit status for yesterday's session — immediately knows to run `/cleanup` (#1790) to check for leftover state.

### Additional context

- OS: Windows 11 Pro (Build 26200 / 24H2)
- Shell: PowerShell 7.5.4
- Copilot CLI: 0.0.420 (win32-x64)
- **`copilot --history` must never invoke the LLM** — it is a pure local read of the session registry. No API calls, no tokens, no premium requests.
- Per-session `events.jsonl` files already exist in `~/.copilot/session-state/` — the registry aggregates from these at session close, requiring no new data collection infrastructure.
- The `/chronicle` command (added in 0.0.419, experimental) generates standups from session history — a queryable registry would give `/chronicle` a richer and more reliable data source.
- Related: #1790 (`/cleanup`) — exit status in the registry (`crashed`, `killed`) feeds directly into cleanup workflows.
- Related: #1784 (Session Stats panel) — the panel shows live per-session stats; the registry is the persistent historical record across all sessions.