tag v3.5.0: unblock CI and repair the test suite

The test plan now runs on every push via a macOS job on the GitHub mirror,
since builds.sr.ht has no macOS image. builds.sr.ht keeps secret scanning and
structure checks, with its stale website/ assertion removed.

Getting there took the suite from ten failures to green. Four were real bugs:
repository descriptions could not be cleared, serviceNotProvisioned errors were
misreported as missing content, code spans rendered their contents as live
markup, and inbox threads sharing a subject on one list collided under
Identifiable.