Adds `gt orphans kill` subcommand that permanently removes orphaned
commits by running `git gc --prune=now`.
Flags:
- --dry-run: Preview without deleting
- --days N: Kill orphans from last N days (default 7)
- --all: Kill all orphans regardless of age
- --force: Skip confirmation prompt
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>