rollout: coalesce thread updated_at touches

[jif-oai]

Why

Metadata-irrelevant rollout events currently refresh threads.updated_at on every flush. That keeps thread recency accurate, but it also turns high-frequency agent output into unnecessary SQLite writes. Recency only needs to advance periodically during an active session, while the final suppressed touch still needs to be persisted before shutdown.

What changed

• coalesce touch-only updated_at writes in the rollout writer, with a short production interval between persisted touches
• retain the latest suppressed touch and flush it during shutdown so the thread is not left stale
• extend rollout recorder coverage for coalesced touches, delayed refresh, shutdown flushing, and the existing missing-thread fallback path

Verification

• Added regression coverage in rollout/src/recorder_tests.rs for coalescing and shutdown flushing behavior.