calm-hub: parallelise GitHub namespace cloning (cloneAll/pullAll)

[jpgough-ms]

GitHubCloneManager.cloneAll() and pullAll() clone/pull every configured namespace sequentially (for (Map.Entry<String, NamespaceRepo> entry : namespaceRepos.entrySet()), GitHubCloneManager.java:66,105). Startup latency is the sum of every repo's clone time.

Fixable cheaply:

• Target directories are disjoint per namespace — nothing shared to corrupt
• GitHubRepoSync is stateless
• A ManagedExecutor is already injected in this package

Fix: run each namespace's clone/pull on the executor in parallel; make the success/failure counters atomic (currently plain int increments).