fix(metrics): Avoid mutating processedFiles array during sorting

Code review feedback addressed array mutation issue:
- Reviewer pointed out that sorting processedFiles in place mutates original array
- User requested using shallow copy to avoid side effects
- Assistant implemented [...processedFiles] spread operator before sorting
- Prevents unintended mutations of input array parameter
- Maintains immutability best practices in function implementation

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/core/metrics/calculateMetrics.ts

@@ -52,7 +52,7 @@ export const calculateMetrics = async (
   const candidateFilesCount = Math.min(processedFiles.length, Math.max(topFilesLength * 10, topFilesLength));
 
   // Get top files by character count first
-  const topFilesByChar = processedFiles
+  const topFilesByChar = [...processedFiles]
     .sort((a, b) => b.content.length - a.content.length)
     .slice(0, candidateFilesCount);