colibri's GPU tiering is OFF unless COLI_CUDA=1 (CUDA/HIP) / COLI_METAL=1 is set — it
is NOT implied by shipping a CUDA binary. We passed CUDA_EXPERT_GB without it, so the
engine aborted at startup: "CUDA_EXPERT_GB requires COLI_CUDA=1" -> exited before READY.

_build_env now defaults the backend on for cuda/hip (COLI_CUDA=1) and metal
(COLI_METAL=1) targets — coderai ships a CUDA build and routes colibri to the CUDA
engine — and only sets CUDA_EXPERT_GB when the CUDA backend is enabled. extra_env is
applied last so COLI_CUDA=0 can force a CPU run.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LoSpEthysqmseCc6Geizty