[maartenbode]

Repositories with many tags or branches that hold no package (common in monorepos) could exhaust the provider's API rate limit: every sync revisited all of those refs, and overlapping syncs queued them several times over.

• Refs without a composer.json at any package path are remembered per commit and skipped until their commit changes. Forced and full syncs still check every ref.
• A sync that starts while an earlier sync's batch is still running is deferred; the batch completion starts one follow-up sync for all requests made meanwhile.
• Rate limits are no longer waited out inside the worker. Providers throw on a rate limit (including GitHub's secondary limits), and ref jobs are released until it resets, with the reset time shared so other jobs of the repository wait without making requests. Waiting no longer counts as a failed attempt.