feat: full action entry point with validation pipeline

[hermanngeorge15]

Summary

• Replaces stub src/index.ts with full GitHub Action entry point
• 4-step validation pipeline: spec-linter -> injection-scanner -> scope checker -> criteria checker
• Builds compliance report, posts/updates single PR comment
• Sets exit code based on fail-on input (errors | warnings | never)
• Sets status and report outputs via core.setOutput()

Verification

• npm run lint -- no type errors
• npm test -- 25/25 tests pass
• npm run build -- dist/index.js produced (965 KB CJS bundle, all deps inlined)