fix: ignore coverage.xml and htmlcov artifacts

Prevents test artifacts leaking into commits (matches mnemo #408 anti-pattern).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: n24q02m

.gitignore

@@ -59,3 +59,8 @@ superpower/
 superpowers/
 docs/superpowers/
 
+
+# Test coverage artifacts
+coverage.xml
+htmlcov/
+.coverage