feat: add model profile ensure hooks

[phaedrus-on-git]

Summary

• add opt-in command-backed model profile ensure hooks before proxying
• return OpenAI-shaped 503 errors when profile preparation fails or times out
• document ensure hooks and use architecture-specific package build outputs

Validation

• go test ./...
• go run ./cmd/devrail-router check -config configs/router.example.yaml
• make package-smoke GOOS=linux GOARCH=amd64
• make package-smoke GOOS=linux GOARCH=arm64
• make docker-smoke
• make vagrant-smoke
• make check