Prune stale tests and fix CI

Review all test suites, delete stale, superseeded, those that were one off to check feature flags or absence of something. Kill the most long running or flaky with unit tests that execute in milliseconds. Regroup between quick and full, Fix broken tests in CI - GitHub Actions - look at a few recent test runs and fix broken/flaky tests. Babysit CI/CD until the tests (test.yaml) pass.

Fixed CI red: regex linearity flake (ratio, mutation-checked), 2 API failures from Task 5884's bootstrap; deleted a shadowed duplicate test. Full gate green; dev deploy ok. test.yml unverifiable until next promotion PR.