archive_session times out after 60 seconds and leaves large worktrees orphaned

`archive_session` can time out while tearing down a large repository worktree, leaving the session and worktree behind without enough information to recover safely. This can consume substantial disk space and prevent reuse of the session branch.

## What happened

1. A native `archive_session` call tried to archive a terminally errored local project session with no file changes. The session used a linked worktree for a large repository.
2. The call returned:

```text
Failed to archive session: git command timed out after 60 seconds
```

3. Retrying once returned the same output.
4. After the first timeout, `get_session` still reported the session, branch, and worktree path, so cleanup had not completed.
5. The specific Git operation that timed out is unknown. The result did not say which operation failed, whether its subprocess was cancelled, which cleanup steps succeeded, or whether retrying or manual cleanup was safe.

The only safe immediate response was to leave the failed session isolated and create another session. That workaround can leave a large worktree on disk and keep its branch in use. The affected Copilot CLI version was not captured.

## Expected

Archiving should either complete for large worktrees or stop cleanly with enough structured state and supported recovery guidance to finish cleanup safely.

## Done when

- [ ] Worktree teardown uses a timeout that scales with repository size, is configurable, or assigns suitable budgets to individual Git operations.
- [ ] A timed-out Git subprocess is actually cancelled and cannot continue changing the worktree after the tool returns.
- [ ] Failure results identify the Git operation, cancellation outcome, completed cleanup steps, and remaining session, branch, and worktree state in a structured form.
- [ ] Cleanup is idempotent and resumable, or a supported cleanup primitive can safely finish a partially completed archive.
- [ ] Documentation explains when retrying is safe, how to inspect partial state, and how to recover without losing work or corrupting Git metadata.

## Related

- https://github.com/github/copilot-cli/issues/4145
- https://github.com/github/copilot-cli/issues/3675
- https://github.com/github/copilot-cli/issues/3518

archive_session のエントリポイントから開始し、そのクリーンアップ状態を、タイムアウト後に get_session が報告する内容と比較します。可能であれば、大規模なリンク済み worktree に対して 60 秒の失敗を再現し、その後、どの Git 操作とクリーンアップ手順が報告されるかを追跡します。完了条件は、タイムアウト処理、構造化された残存状態、安全な再試行またはリカバリ、そして部分アーカイブ時の動作の文書化です。

索引モデルが issue の本文から書いたものです。