fix: remove unused imports and variables

- Remove unused SEVERITY_LEVELS import from AnalyzeProject.ts
- Prefix unused fileName variable with underscore in HardcodeDetector.ts
- Replace || with ?? for nullish coalescing

packages/guardian/src/application/use-cases/AnalyzeProject.ts

@@ -19,7 +19,6 @@ import {
     REGEX_PATTERNS,
     REPOSITORY_VIOLATION_TYPES,
     RULES,
-    SEVERITY_LEVELS,
     SEVERITY_ORDER,
     type SeverityLevel,
     VIOLATION_SEVERITY_MAP,