chore(meta): update Makefile and gitignore for Godot client CI

- Implement lint-client, build-client, test-client Make targets
- Add GODOT variable for binary detection
- Gitignore .godot cache, export artifacts, test reports
- Clean target handles Godot artifacts

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -1,6 +1,12 @@
 # Build and cache
 .cache/
 
+# Godot
+client/.godot/
+client/export/
+client/reports/
+client/*.import
+
 # Database journal files (transient, not the DB itself)
 db/commonwealth.db-wal
 db/commonwealth.db-shm