Wire orphaned test files into check.sh gate

Found while surveying the repo for code reduction (Task 11727): `tools/check.sh`'s `pytest_stage` admits one file per stage and nothing collects a directory, so these 14 test files are never run by any tier, workflow, `dc` or packaging step (grep of their basenames across tools/check.sh, .github/, dc, packaging/ is empty). Their subjects still exist, several were edited in October 2026 (Tasks 11113, 11272), and they may already be red since nothing has run them. - infra/host/gha-runners/test_gha_job.py (752 lines), test_ephemeral.py (456), test_slot_user_removal.py (289) — the neighbouring test_install_*.py files ARE listed at tools/check.sh:1587-1594. - infra/jaah/test_pkgmgr.py (156), test_ci_trust_watch.py (148), test_data_region_bucket.py (98), test_instance_browse_grants.py (39). - tools/tests/test_postinst_selinux.py (181), test_postinst_jaah_home_links.py (157), test_lock_jaahd_requirements.py (121), test_build_jaah_python.py (97), test_postinst_daemon_gitconfig.py (88), test_jaah_python_gate.py (84), test_postinst_cli_prefix_links.py (75). Ask: run each file, fix or quarantine what is red per docs/standards/coding-standards.md §Flake quarantine, and add each to the right tier in tools/check.sh per its header's admission rules (quick vs run_bg_full). Add one meta-check (in tools/check.sh's own selftest or a small guard) that every test_*.py under tools/tests, infra/jaah and infra/host/gha-runners is named by some stage, so this cannot recur. Delete a file only when its subject is gone (prove it: the subject import fails). Out of scope: changing what the tests assert; touching apps/api or apps/web test collection (those suites are collected by directory already). Done means: - `grep -c` of each basename above in tools/check.sh is >= 1, or the file is deleted with its subject proven gone. - The new meta-check is in the quick tier and passes. - `./dc check quick --gate-result` reports status=pass. Required checks: ./dc check quick

Wired 23 orphaned test files (the task's 14 + 9 more the guard found) into check.sh tiers; new quick meta-check tools/check_test_stages.py; fixed the real regression test_jaah_python_gate caught in accounts.py. Quick gate pass; PR #7465 queued. No UI surface. Memory: none applied.