[Duang777]

Summary

• Pass the branch name after -- in the detached background force-delete fallback.
• Add an end-to-end regression test for a plumbing-created -x branch when the staged rename fails.

Root cause

The rename-failure fallback generated git branch -D <branch>. Git interpreted a valid flag-like ref as an option, so the worktree disappeared but the branch remained.

Tests

• Confirmed the new regression test timed out on the old command while waiting for refs/heads/-x to be deleted.
• Confirmed the regression test and all four background fallback integration tests pass after the fix.
• cargo test test_build_remove_command
• cargo run -- hook pre-merge --yes (4964 passed, 1 skipped)