chore: gitignore Flutter/Dart build artifacts and IDE files, untrack committed .dart_tool

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -20,4 +20,11 @@ __pycache__/
 
 # 빌드/배포 HTML 산출물
 .agents/skills/multi-agent-mux-delegate-job/USER_MANUAL.html
-.agents/skills/multi-agent-mux-delegate-job/mqtt-broker-setup.html
+.agents/skills/multi-agent-mux-delegate-job/mqtt-broker-setup.html
+
+# Flutter/Dart 빌드 산출물 및 IDE 파일
+.dart_tool/
+build/
+.idea/
+*.iml
+ephemeral/