refactor(cli): make configuration validation side-effect free

[luojiyin1987]

Summary

• Add structured CLI errors for configuration validation.
• Make configuration utilities throw errors without process side effects.
• Format validation errors at the CLI boundary.
• Preserve current messages, stderr output, and exit status.

Part of #131.

Behavior

Commander still owns parser errors. Invalid JSON still includes the original parse error.

Validation

• npm run lint
• npm test -- --runInBand
• npm run test:package

All 167 tests pass.