v5.1.0: m Add sync --if-stale to collect only when the mirror is stale

sync --if-stale skips collection when the last successful collection is
newer than stale_after (config, default 1h; --stale-after overrides it for
one call), or when another collector already holds the mirror writer lock
with a live session. Skips exit 0 with skipped, reason, collected_at, age
and stale_after, and make no GitHub requests, index or publish. Stale,
never-collected and missing mirrors sync as before; an unowned interrupted
session resumes. The age check repeats inside the collection transaction so
concurrent callers never both collect. Plain sync is unchanged.

Status.CollectionAge is now shared by doctor and sync. Update the README,
spec, AGENTS.md and example configuration.