refactor(cli): centralize lint failure decision

[luojiyin1987]

Summary

• Add the pure shouldFailLint function.
• Use one failure policy for stdin and file lint.
• Keep errors and rule failures fatal.
• Suppress warnings only when the option is active.

Part of #131.

Tests

The policy test covers all 16 input combinations. It varies errors, warnings, rule failures, and warning suppression.

Validation

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

All 183 tests pass.