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

source/game_render.cpp

@@ -164,21 +164,18 @@ void draw_game_over_screen(const GameState &state) {
   iprintf("\x1b[8;4HDifficulty:    %s", state.hardMode ? "Hard  " : "Normal");
 
   iprintf("\x1b[10;4HCause of Defeat:");
-  const char *cause = "Collision";
-  switch (state.gameOverReason) {
-  case GameOverReason::SelfCollision:
-    cause = "Bite Collision (Hit Body)";
-    break;
-  case GameOverReason::BarrierCollision:
-    cause = "Energy Barrier Impact";
-    break;
-  case GameOverReason::RottenAppleDepleted:
-    cause = "Depleted by Rotten Apple";
-    break;
-  default:
-    cause = "Collision";
-    break;
-  }
+  const char *const cause = [&state]() {
+    switch (state.gameOverReason) {
+    case GameOverReason::SelfCollision:
+      return "Bite Collision (Hit Body)";
+    case GameOverReason::BarrierCollision:
+      return "Energy Barrier Impact";
+    case GameOverReason::RottenAppleDepleted:
+      return "Depleted by Rotten Apple";
+    default:
+      return "Collision";
+    }
+  }();
   iprintf("\x1b[11;4H  %s", cause);
 
   iprintf("\x1b[14;2H----------------------------");