Add `gt convoy close` command to manually close convoys regardless of
tracked issue status. This addresses the desire path identified in
convoy-lifecycle.md.
Features:
- Close convoy with optional --reason flag
- Send notification with optional --notify flag
- Idempotent: closing already-closed convoy is a no-op
- Validates convoy type before closing
Closes hq-2i8yw
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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