Pure retention policy: decide which annotations have aged out.

Only terminal annotations (:resolved/:dismissed) can expire, and only once older than the configured TTL — measured from resolved_at, falling back to inserted_at. Open annotations (:pending/:acknowledged) are never expired regardless of age, so unhandled feedback is never silently dropped.

This module holds no state and reads no clock: callers pass now, which keeps the policy trivially testable and lets the orchestrator run it on hydrate.