Refresh the model catalog from models.dev

Unforced calls skip the network when the last models.dev check is younger than 4h (skipped-fresh). force: true always fetches, still conditional on the stored ETag (not-modified on 304), but a forced call within a minute of the previous one returns skipped-cooldown with retryAfterMs. Concurrent refreshes share one fetch. Lead agent or operator only. added lists provider/modelId keys new since the previous fetch.