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/dashboard/app/components/SettingsModal.tsx

@@ -454,7 +454,7 @@ export function SettingsModal({
               <input
                 id="taskPrefix"
                 type="text"
-                placeholder="KB"
+                placeholder="FN"
                 value={form.taskPrefix || ""}
                 onChange={(e) => {
                   const val = e.target.value;