Fix flaky test tests/test_git_mirror_isolation.py::test_a_planted_key_steers_no_daemon_git[create_worktree-local-url]

The merge lane saw `tests/test_git_mirror_isolation.py::test_a_planted_key_steers_no_daemon_git[create_worktree-local-url]` fail and then pass on a rerun of the same tree. - Test: `tests/test_git_mirror_isolation.py::test_a_planted_key_steers_no_daemon_git[create_worktree-local-url]` - Tree: `97f5c44e1a99d35c0c115ffec4f12ddbf7381add` - Lane task: 7584 - Flake record: 01a151b7-9bf8-78c0-93a5-c86b526c3de7 Make it deterministic; do not delete it unless it tests nothing.

Fixed the flake's root cause: conftest FakeBroker.close() left its accept thread alive, so the thread stole the mirror test's TLS handshakes on a reused fd. It now joins the thread before closing. Added a red→green regression test. Quick gate and 705 changed-area tests green. PR #6117 queued.