feat(KB-334): change default task prefix from KB to FN

- Change default taskPrefix from KB to FN and branch naming from kb/ to fusion/
- Update all test expectations and patterns for new naming convention
- Update settings UI placeholder text to reflect FN prefix
- Update dashboard, engine, and CLI components for fusion/ branch paths
- Add changeset documenting the breaking change for users

packages/cli/src/__tests__/task-steer.test.ts

@@ -66,7 +66,7 @@ describe("runTaskSteer", () => {
 
     expect(mockAddSteeringComment).toHaveBeenCalledWith("FN-001", "Focus on error handling", "user");
     expect(mockConsoleLog).toHaveBeenCalledWith(
-      expect.stringContaining("Steering comment added to KB-001")
+      expect.stringContaining("Steering comment added to FN-001")
     );
   });