KTD1 first step (in-repo decoupling, behavior preserved):
- squad.Options{BackendName, BackendTimeout, DataDir, NewBackend} replaces
*config.Config; defaults applied in withDefaults
- backend construction injected by caller (bizdev assembly): credentials,
environment and concurrency stay product-side; nil factory fails closed
instead of silently degrading to Stub
- verified: go vet ./... clean, squad/bizdev short tests pass