feat(runtime): centralize provider selection and direct fallback

[Patel230]

Summary

• centralize preferred-provider resolution inside Eyrie runtime
• infer provider from model when Hawk only provides model intent
• construct direct OpenAI to Anthropic fallback inside runtime transport
• add runtime transport policy coverage for provider preference and fallback

Verification

• go test ./runtime -count=1
• go test -race ./runtime -count=1
• golangci-lint run ./runtime --timeout=5m
• pre-push hook: govulncheck, full test suite, go vet