fix(handler): filter command files and guard non-command modules

Handler.ts iterated readdirSync(COMMANDS_PATH) without filtering, so
any .d.ts declaration file, .js.map source map, or non-class module
that landed in commands/ would crash startup with 'mod.default is not
a constructor'. Restrict loading to .ts/.js (excluding .d.ts and maps)
and skip modules without a default-exported constructor.

src/classes/Handler.ts

@@ -8,9 +8,17 @@ export class Handler {
   public coolDown: Map<string, number> = new Map();
 
   public constructor() {
-    const commandFiles = readdirSync(COMMANDS_PATH);
+    const commandFiles = readdirSync(COMMANDS_PATH).filter((f) => {
+      // ts-node로 dev 실행 시 .ts, 빌드 후엔 .js. 둘 다 허용하고
+      // 선언 파일(.d.ts)/소스맵(.js.map)은 제외한다.
+      if (f.endsWith(".d.ts")) return false;
+      if (f.endsWith(".js.map") || f.endsWith(".ts.map")) return false;
+      return f.endsWith(".ts") || f.endsWith(".js");
+    });
     for (const commandFile of commandFiles) {
-      const command = new (require(COMMAND_PATH(commandFile)).default)() as Command;
+      const mod = require(COMMAND_PATH(commandFile));
+      if (typeof mod.default !== "function") continue; // not a command module
+      const command = new mod.default() as Command;
       this.commands.set(command.metaData.name, command);
     }
   }