Add WorkManager guidance and navigation exception

Add a new "Background Tasks & WorkManager" section with checklist items for reviewing background task code (app background/killed behavior, doze/timing, minimum periodic interval, background location permissions, battery optimization, data passed to background isolates, failure handling, and task cleanup). Also clarify navigation guidance by allowing Navigator.push() when popping a dialog (note added to both Mobile Architecture skill and Mobile QA Analyst agent docs).

.agent/skills/krow-mobile-architecture/SKILL.md

@@ -511,7 +511,7 @@ Modular.to.popSafe();
 
 // ❌ AVOID
 Modular.to.navigate('/home');  // No safety
-Navigator.push(...);            // No Modular integration
+Navigator.push(...);            // No Modular integration (except when popping a dialog).
 ```
 
 ### Data Sharing Patterns