chore(acq): clean up leftovers from the host-authority lifecycle migration #650

Problem

Small leftovers from the host-authority lifecycle migration (#637) that are harmless but should not rot:

1. scripts/test-acq-lib.sh: STUB_AGENT_USER_READY and seed_host_config_gates seed an agent-user-ready host key that nothing in acq reads any more (the agent-user setup is idempotent and runs on every provision and heal). Four tests still use them. Remove the dead seeding and adjust the tests.
2. docs/adr/0011-msb-backend-and-neutral-kits.md still describes the agent-user setup as marker-gated (historical ADR). Add a short note pointing to ADR-0035's follow-up paragraph, or leave a superseded-by line.
3. docs/KNOWN_FAILURE_MODES.md has duplicate section 43 headings (already tracked in Fix duplicate section 43 headings in KNOWN_FAILURE_MODES #641; do not duplicate that work).
4. scripts/qualify-msb-ro-mount is now an optional, non-gating check (about 250 lines plus 14 tests). Decide whether to keep it for upgrade checks or delete it as dead weight.

Acceptance

Each item is either done or explicitly closed as keep; tests pass; docs stay accurate.

Child of #503 (follow-up). AI-assisted tracker entry (OpenCode). Human review required.