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/core/src/__tests__/setup-test-isolation.test.ts

@@ -1,5 +1,7 @@
 import { describe, expect, it } from "vitest";
 import { homedir, tmpdir } from "node:os";
+import { dirname, join, resolve } from "node:path";
+import { fileURLToPath } from "node:url";
 
 const TEMP_HOME_PREFIX = "fn-test-home-";
 
@@ -26,4 +28,20 @@ describe("test isolation setup", () => {
     expect(dir).toContain(tmpdir());
     expect(dir).toMatch(/fn-test-home-.*[\\/]\.fusion$/);
   });
+
+  it("GlobalSettingsStore() without explicit dir throws under VITEST guard", async () => {
+    const { GlobalSettingsStore } = await import("../global-settings.js");
+
+    expect(() => new GlobalSettingsStore()).toThrow(
+      "resolveGlobalDir() called without explicit dir during test execution. Pass a temp directory to avoid writing to real ~/.fusion/",
+    );
+  });
+
+  it("cwd is not inside the repository .fusion directory", () => {
+    const thisFile = fileURLToPath(import.meta.url);
+    const repoRoot = resolve(dirname(thisFile), "../../../../");
+    const repoFusionDir = join(repoRoot, ".fusion");
+
+    expect(process.cwd().startsWith(repoFusionDir)).toBe(false);
+  });
 });