feat(FN-2606): merge fusion/fn-2606 (auto-resolved)

- fix(FN-2606): complete Step 4 — clean up mkdtemp test directories
- test(FN-2606): complete Step 3 — add isolation guard coverage
- feat(FN-2606): complete Step 2 — add engine/dashboard HOME test isolation setup

packages/dashboard/src/__tests__/setup-test-isolation.ts

@@ -0,0 +1,12 @@
+/**
+ * Global test isolation: prevents dashboard tests from writing to the real ~/.fusion/ directory.
+ *
+ * This runs in every Vitest worker before shared setup. By forcing process.env.HOME
+ * to a fresh temp directory, homedir()-derived paths resolve to isolated locations.
+ */
+import { mkdtempSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+
+const tempHome = mkdtempSync(join(tmpdir(), "fn-test-home-"));
+process.env.HOME = tempHome;