Wire baseline and CI gate policy into run

[cervantesh]

Summary

• evaluate existing baseline and CI gate flags in pkg/engine and expose additive gate results on RunResult
• fail cervomut run only after reports are written, using the shared gate evaluation instead of a CLI-only fail_under check
• document and test baseline skip/error semantics plus gate output in summary, JSON, HTML, and GitHub summary

Testing

• go test ./pkg/engine ./cmd/cervomut ./pkg/report
• go test ./...

Refs #368