Follow-ups on the #85006 salvage:
- A key_cmd token with no advertised expiry was cached for the life of the
process. The "refresh on 401" contract it relied on has no implementation
(SDK retries cover 429/5xx only), so an expired no-TTL token would 401
every request until restart. Cache on a bounded 15-minute window instead;
helpers that want a longer cache can advertise their real expiry.
- Test for the no-TTL path updated to pin the bounded-window contract;
the remint test's $RANDOM (bash-only, empty under dash) replaced with
date +%s%N so it exercises remint under any /bin/sh.
- website/docs/integrations/providers.md: document key_cmd in the named
custom providers section (contract, precedence, secrets.command contrast).
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