fix(ci): clean up clang-format artifacts before REUSE check

The clang-format-action creates a failing-files.txt file when it detects
formatting issues. Since we use continue-on-error for this step, the file
persists and causes the REUSE compliance check to fail. Delete this file
before running REUSE check.

Also fix corrupted header in coverage.yml and update .reuse/config to
exclude temporary/local files from compliance checks.

Author: ahojukka5

.github/workflows/ci.yml

@@ -44,6 +44,9 @@ jobs:
           echo "Please run 'clang-format -i' on modified files locally."
           echo "This is a warning only and will not block the build."
 
+      - name: Clean up clang-format artifacts
+        run: rm -f failing-files.txt
+
       - name: Check REUSE compliance
         uses: fsfe/reuse-action@v2