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

AGENTS.md

@@ -50,13 +50,13 @@ snakePlus/
 │
 ├── sounds/                     # Audio assets (built into soundbank by mmutil)
 │   ├── 0rigin.xm               # FastTracker II background music track (MOD_0RIGIN)
-│   ├── 273792__pacomav__plop.wav                                  # Movement plop SFX
-│   ├── 332629__treasuresounds__item-pickup.wav                    # Apple pickup SFX
-│   ├── Boom.wav                                                   # Rotten / Cursed apple boom SFX
-│   ├── denied.wav                                                  # Reverse-direction denied thud SFX (synthesized)
-│   ├── 167126__crisstanza__unpause.wav                            # Unpause SFX
-│   ├── 167127__crisstanza__pause.wav                              # Pause SFX
-│   └── 73750__timbre__remix-of-benboncan-sad-trombone-more-wah-bright-de-clicked.wav # Fail SFX
+│   ├── Boom.wav                # Rotten / Cursed apple boom SFX (SFX_BOOM)
+│   ├── denied.wav              # Reverse-direction denied thud SFX (SFX_DENIED)
+│   ├── pause.wav               # Pause SFX (SFX_PAUSE)
+│   ├── pickup.wav              # Apple pickup SFX (SFX_PICKUP)
+│   ├── plop.wav                # Movement plop SFX (SFX_PLOP)
+│   ├── sad_trombone.wav        # Fail SFX (SFX_SAD_TROMBONE)
+│   └── unpause.wav             # Unpause SFX (SFX_UNPAUSE)
 │
 ├── tools/                      # Tooling & automation scripts
 │   └── clang_tidy_watch.sh     # File watcher script running clang-tidy on file change
@@ -72,7 +72,8 @@ snakePlus/
 │
 ├── .clang-tidy                 # Clang-tidy rules and naming conventions
 ├── .clangd                     # Clangd LSP configuration and system include paths
-└── .gitignore                  # Git ignore rules for build artifacts and caches
+├── .gitignore                  # Git ignore rules for build artifacts and caches
+└── codechecker_skip.txt        # Skipfile for CodeChecker to omit external devkitPro headers
 ```
 
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