Hardens #274 branches-only guard for gitea_create_issue:
- 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 threaded into preflight guard.
- tests/test_create_issue_workspace_guard.py: stable-control-checkout rejection
and invalid-path fail-closed cases (PROJECT_ROOT simulated as control checkout).
Preserves WIP commit 8530109 implementation; test fix completes stable-checkout path.
Closes#450
