Follow-up on the branches-only worktree guard (#274):
- verify_preflight_purity: validate resolved worktree exists, is a directory,
and belongs to the target repository before author mutations.
- gitea_create_issue: add worktree_path for explicit branches/ workspace proof.
- tests/test_create_issue_workspace_guard.py: six regression cases including
stable-control-checkout rejection with explicit PROJECT_ROOT simulation.
Closes#274
