Copilot CLI never prunes ~/.copilot/session-state, causing EMFILE / file-descriptor exhaustion (crashes VS Code Copilot Chat)

### Summary

The Copilot CLI creates a `~/.copilot/session-state//` folder for **every** session — including background automations, heartbeats, and memory-consolidation runs — and **never prunes them**. On a heavily-used machine these accumulate into the thousands. A Copilot component then bulk-opens `session-state/*/workspace.yaml`, exhausting the process's file-descriptor table (libuv's ~8189 limit on Windows) and throwing `EMFILE: too many open files`. In VS Code this makes Copilot Chat hang on "working" and crash after a few turns.

This is the root cause behind microsoft/vscode#313799 — that's where it *manifests*, but the fix belongs here.

### Evidence (Windows, measured with Sysinternals `handle.exe`)

- `~/.copilot/session-state` contained **10,313 `workspace.yaml` files**, accumulated since 2026-02-18, never pruned.
- The Copilot `node.mojom.NodeService` utility process peaked at **~8,788 open handles** — right at the libuv fd ceiling — then dropped back to ~580 after the burst (which is why point-in-time snapshots look normal).
- In a single mid-spike handle dump, **3,256 of 3,352 (97%) File-handle paths were `session-state\...\workspace.yaml`**.
- Reducing the folder count to ~67 **immediately** resolved the EMFILE crashes.

### Two underlying issues

1. **No retention/cleanup (primary):** session-state folders grow unbounded — no age or count cap, no garbage collection.
2. **Unbounded fan-out open:** the code that reads `workspace.yaml` across sessions opens them without bounding concurrent file descriptors or closing promptly, so a large directory exhausts the fd table instead of degrading gracefully.

### Suggested fixes

- Add automatic retention (e.g., prune session-state older than N days, or cap to the M most-recent) on CLI startup and/or via a background task.
- Bound concurrency and ensure prompt handle closure when enumerating session metadata.
- Optionally consolidate session metadata into a single index/DB rather than one folder + file per session.

### Workaround

Periodically prune `~/.copilot/session-state` (e.g., a scheduled task that deletes folders older than ~14 days).

### Environment

Windows; GitHub Copilot CLI + VS Code Copilot Chat (stable).

Beginne damit, den CLI-Code zu lokalisieren, der ~/.copilot/session-state erstellt und liest, und verfolge dann die Einstiegspunkte für den Start oder Hintergrundaufgaben sowie die Aufzählung von workspace.yaml. Reproduziere den Fall einer großen Sitzung unter Windows und überprüfe, dass die Aufbewahrung alte Zustände entfernt, Dateiles Vorgänge zeitnah geschlossen werden und die CLI unter der gemeldeten Arbeitslast nicht mehr EMFILE erreicht.

Vom Indexierungsmodell aus dem Issue-Text verfasst.