[codex] Add CI workflow and unify build entrypoints

[Arlowen]

What changed

• add a standard CI workflow for push and pull request verification
• expand Makefile into the single local entrypoint for test, vet, test-race, cover, ci, build, and clean
• update scripts/all_build.sh to reuse make test and make build
• align the release workflow test step with the same make test entrypoint

Why

The repository previously only verified changes during release, and local build/test entrypoints were split across multiple places. This change adds regular validation on non-tag pushes and pull requests, and reduces drift by making CI, release, and local scripts share the same build/test commands.

Impact

• contributors get earlier feedback from CI before tagging a release
• coverage is reported using -coverpkg=./... so it reflects the current test/... package layout
• local build automation now has a single source of truth in Makefile

Validation

• make ci
• bash scripts/all_build.sh