/cleanup slash command — detect and resolve orphaned, stale, and hung processes from previous sessions

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

When a Copilot CLI session crashes, is force-killed, or exits uncleanly, it can leave behind a range of residual state: orphaned child processes, hanging MCP server processes, stale lock files, lingering worker or sub-agent processes, and session database files in an inconsistent state. Currently there is no built-in way to detect or resolve any of this. Users who experience repeated crashes or unexpected behaviour across sessions have no tool to audit or clean up leftover state — they must either reboot, hunt for processes manually, or live with the side effects.

The fix must be **lightweight and token-free** — a pure local operation that does not invoke the LLM, consume API quota, or count as a premium request.

### Proposed solution

Introduce a **`/cleanup`** slash command that performs a fast, local-only scan of all residual state from previous sessions and presents a concise, actionable report — without invoking the model or consuming any API quota.

**What it scans:**

| Category | What it looks for |
|---|---|
| **Orphaned processes** | Child processes (MCP servers, sub-agents, worker tasks) whose parent session PID no longer exists |
| **Hung / unresponsive processes** | Copilot CLI processes that are running but not responding (detectable via PID status + lock file age) |
| **Stale lock files** | `.lock` or session mutex files left behind by crashed instances |
| **Inconsistent session state** | `session.db` files from sessions that never cleanly closed (no clean-exit marker) |
| **Abandoned log files** | Oversized or orphaned log files from sessions that did not rotate cleanly |
| **Zombie sub-agents** | Background agent processes that are still alive but whose parent session is gone |

**Output format — compact, structured, actionable:**

```
/cleanup — Session Health Check

✖ 2 orphaned MCP server processes (playwright PID 18842, git PID 19001)
✖ 1 stale lock file (~/.copilot/session-state/813d.../session.lock age: 4h 12m)
⚠ 1 inconsistent session DB (~/.copilot/session-state/813d.../session.db)
✔ No hung Copilot CLI processes found
✔ No zombie sub-agent processes found

Found 3 issue(s). Resolve all? (Y/n) — or press Enter to review individually
```

**Resolution options (per item or all-at-once):**
- **Kill** orphaned/hung processes (`SIGTERM` → grace period → `SIGKILL` / `Stop-Process` on Windows)
- **Delete** stale lock files
- **Archive or delete** inconsistent session DBs (with option to attempt recovery first — see #1780)
- **Rotate** oversized log files

**No LLM involvement — this is entirely a local process/filesystem operation.** It must not trigger any API call, token usage, or premium request.

**Additional flags for automation:**
```
/cleanup --dry-run # Report issues without taking any action
/cleanup --force # Resolve everything without confirmation prompts
/cleanup --stale-only # Only target lock files and session state, skip processes
```

### Example prompts or workflows

1. User returns after a machine sleep that killed their session. Runs `/cleanup` — two orphaned MCP servers and a stale lock file are found and resolved in seconds, zero tokens spent.
2. User notices a new session starts slowly. `/cleanup --dry-run` reveals a hung Copilot CLI process from a previous crash is still holding resources.
3. CI/CD script runs `/cleanup --force` before starting a fresh session to guarantee a clean environment with no residual state from a previous run.
4. User runs `/cleanup` and finds an inconsistent session DB. Chooses to attempt recovery (hands off to #1780 queue recovery flow) rather than delete it.
5. `/cleanup` finds nothing — outputs `✔ All clear. No orphaned or stale state found.` and exits immediately, consuming no time or tokens.

### Additional context

- OS: Windows 11 Pro (Build 26200 / 24H2)
- Shell: PowerShell 7.5.4
- Copilot CLI: 0.0.420 (win32-x64)
- **This command must never invoke the LLM** — it is a local diagnostic and remediation tool only. No API calls, no token usage, no premium requests consumed under any circumstances.
- `/diagnose` already handles connectivity checks — `/cleanup` is the complementary local health check for process and session state.
- On Windows, process termination should use `Stop-Process` / `TerminateProcess`; on Unix, `SIGTERM` followed by `SIGKILL` after a grace period.
- The session state directory (`~/.copilot/session-state/`) and log directory (`~/.copilot/logs/`) are the primary scan targets — both are already well-defined locations.
- Related: #1780 (queue recovery from crashed sessions) — `/cleanup` and queue recovery are complementary; `/cleanup` handles processes and locks, #1780 handles prompt data.