A stored queue secret with no `minted_at` counted as due, and no secret
minted before the renewal pass has one, so the first pass after deploy
would re-mint every agent's secret. Every reconnect before that agent's
next restart would then be refused.
Such a secret is now stamped instead: `minted_at = now` is written beside
the unchanged `value`, and its 45-day clock starts there. Only a secret
whose recorded mint time is at least 45 days old gets a new value.
The decision is `secret_step` (Keep / Backfill / Remint), and the pass
reports an unstamped secret as `Observed::Unstamped`. Backfill and
re-mint log different lines.
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