Add opencode smoke coverage in CI

[moolen]

Summary

• run a repo-owned opencode smoke suite in CI across proxy/transparent, audit/enforce, explicit env/copy_env, and docker_build permutations
• synthesize an OpenAI provider config so opencode deterministically fails on missing credentials inside bbox
• keep buffered stdin runs non-interactive so opencode run works in the buffered CLI path

Verification

• go list ./... | grep -v ^github.com/moolen/bbox/integration | xargs go test -count=1 -timeout=20m
• ./scripts/opencode-smoke.sh