[codex] Add v0.2.1 stability checks

[ryoreonhz-dev]

Summary

• Fix safe model replacement so provider model names containing /, \, or & can be written without corrupting config.toml.
• Add stricter mikefarah/yq v4 detection and clearer diagnostics for custom registries.
• Add generic sync <provider> safety checks to avoid overwriting provider baselines with unrelated active configs.
• Add shell smoke tests and a GitHub Actions workflow for syntax and behavior coverage.

Validation

• bash -n scripts/codex-toggle
• bash -n tests/smoke.sh
• bash tests/smoke.sh
• git diff --check

Notes

The workflow file was created through the GitHub connector because the local PAT used by git push does not have the workflow scope required to introduce .github/workflows/smoke.yml. The final branch was then rebased onto that connector-created workflow commit and pushed normally.

[ryoreonhz-dev]

Agree