mirror push: PushParams.Parallelism is unused; push is sequential

Summary

d8 mirror push uploads images sequentially, but PushParams.Parallelism is populated and never read — it is dead configuration. This also makes the README "Best Practices → Parallel Operations" note misleading for push.

Details

• internal/mirror/cmd/push/push.go sets Parallelism: params.ParallelismConfig{Blobs: ..., Images: ...}.
• pkg/libmirror/operations/params/push.go defines ParallelismConfig and DefaultParallelism.
• No code reads .Parallelism (only the definition and the assignment reference it), and the push path contains no concurrency primitives (go func, errgroup, sync.WaitGroup). internal/mirror/pusher/pusher.go pushes in a plain sequential for loop.

Impact

Dead config; pushes of large bundles are slower than the config suggests, and documentation implying parallel push is inaccurate.

Expected

Either wire the Parallelism config into the pusher, or remove the unused config and adjust the docs.

Severity: low (cleanup / performance). As of main (62132d6a).

Start with internal/mirror/cmd/push/push.go, pkg/libmirror/operations/params/push.go, and internal/mirror/pusher/pusher.go to trace the configuration assignment and sequential push loop. Review the README Best Practices → Parallel Operations note, then determine whether the configuration should be wired in or removed; done means the implementation and documentation no longer contradict each other.