tests(test_worktree) Add tests for remaining coverage gaps (Phase 3)

why: Improve test coverage from 94% toward 98%.
what:
- Add WorktreeActionOutputFixture for color branch coverage (lines 227-231)
- Add test_sync_worktree_unchanged_execution (lines 549-552)
- Add test_sync_worktree_oserror_exception (lines 557-559)
- Add test_handle_list_with_empty_worktrees_direct (line 201)
- Add test_handle_sync_with_empty_worktrees_direct (line 288)
- Add test_cli_prune_no_existing_worktrees (line 356)
- Add test_get_worktree_head_oserror (lines 305-306)

Author: tony