chore: gitignore 截图验收产物目录（.screens/.tmp-screens）

.gitignore

@@ -5,6 +5,10 @@
 # Local development docs
 /DevDocs/
 
+# Local acceptance screenshots (screenshot channel output, never committed)
+/.screens/
+/.tmp-screens/
+
 # Agent workspace (local-only)
 /.tower/