A single provider rate-limit (HTTP 429) or overload (503/529) on one
design sub-agent request had no recovery: no client-side pacing to stay
under the RPM limit, and no backoff-retry once tripped — so the section
burned its attempts and the run reported a failed subtask while the rest
of the design was fine. Add a process-wide min-gap throttle (default
350ms, env-overridable) and transparent backoff-retry that honors
Retry-After (else exponential 1/2/4s, capped) around both the openai-
compatible and anthropic send paths. Benefits every builtin path
(orchestrator, design loop, chat). retry.rs stays unchanged: 429 remains
non-retryable at the ladder level since the http layer already backed off.
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