feat(KB-332): rename task prefix from KB to FN and branches from kb/ to fusion/

- Change default task prefix from KB to FN across all packages

- Rename branch naming pattern from kb/{id} to fusion/{id}

- Update all test assertions and fixtures to use new prefixes

- Add changeset for the breaking change

- Resolve merge conflicts in TaskCard.test.tsx touch gesture tests

packages/engine/src/merger.ts

@@ -538,7 +538,7 @@ export async function aiMergeTask(
     );
   }
 
-  const branch = `kb/${taskId.toLowerCase()}`;
+  const branch = `fusion/${taskId.toLowerCase()}`;
   const worktreePath = task.worktree;
   const result: MergeResult = {
     task,