feat: add diff and file metadata capture

[SydilSohan]

Summary

• Fetch PR files via GitHub API with metadata (filename, status, additions, deletions)
• Fetch full unified diff via GitHub API
• Add include_diff input (default: true) to optionally disable diff capture
• Send files array and diff to Pie API
• The API uploads diff to GCS and returns diffGcsPath

Debug Output Added

• PR metadata (repo, PR number, title, author, branches)
• Files changed with status and line counts
• First 500 chars of diff
• Full API response

Changes

• action.yaml - Main action logic with diff capture
• README.md - Updated docs with new features
• .github/workflows/test-codechanges.yml - Test workflow with debug output

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