moltenhub-run-multi-repo-clones-concurrently

[jefking]

Summary

• run repository clone stage concurrently when multiple repos are selected
• preserve existing fallback-to-default-branch behavior per repo and keep branch resolution consistent
• keep sequential fetch-main behavior for non-main base branch reuse after clone completion
• make harness tests clone-order resilient when concurrency is enabled
• add a concurrency barrier test proving both clone commands execute in parallel

Validation

• go test ./... -count=1
• go test -race ./internal/harness -count=1