Fix and prune CI test suites

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.

PR #5945 queued to merge lane (mem-2 used): CI reds fixed (task-page cache, NodeHealth fixture, org-switcher race, org-limits), 5 dead-subject tests deleted, 2 slow tests ~3s→0.07s/4.3s→1.9s. Deferred: agent-session /end node-ownership gap (security), 3 develop-owned API reds.