ci: classic API guard on feat-cmux-next (warns only)
Summary
The feat-cmux-next copy of #35081's classic API guard. Leo, 2026-11-04: "apparently the api if we change it it'll auto deploy so we wanna make extra sure we dont change it for clasisc".
Into feat-cmux-next the check only warns. This branch deploys nothing classic: its merge-base with main is 2026-10-19 and nothing merges or syncs one into the other. The warning keeps such changes visible in case the branches are merged for a release. The files are identical to #35081's head (53026e9a); the test is registered in tests/test-execution.toml and run from ci-guards.yml.
Testing
python3 tests/test_classic_api_guard.py (13 OK), test_ci_warm_sources.py, test_ci_label_events_skip_full_ci.py, test_ci_runs_on_values.py and validate_test_execution_registry.py --base-sha origin/feat-cmux-next: all pass locally.
Observed / Not run
Observed: the tests above. Not run: a live PR event.
Changelog
none
Agent session: agent session