fix: resolve CodeChecker findings and add CodeChecker ignore rules

- Refactor cause in draw_game_over_screen to an immediately invoked lambda, eliminating dead store warning
- Rename sound effect files to standard-compliant names without double underscores to prevent reserved macro warnings in soundbank.h
- Update source/game_audio.cpp to use clean SFX_* macro identifiers
- Add codechecker_skip.txt to ignore external devkitPro toolchain headers
- Ignore .codechecker/ output artifacts directory in .gitignore
- Update AGENTS.md file tree and sound asset references

.gitignore

@@ -55,6 +55,7 @@
 # Editor/LSP tooling caches
 /.cache/
 /.clang-tidy-db/
+/.codechecker/
 
 # CMake internals
 CMakeCache.txt