test(FN-2122): isolate HOME for core and CLI vitest runs

- Add test setup files in core and CLI that override HOME to a per-worker temp directory
- Wire the new isolation setup into core and CLI vitest setupFiles before existing test bootstrap
- Add a core canary test to verify HOME, homedir(), and defaultGlobalDir() resolve under isolated temp paths
- Document how global HOME isolation complements per-fixture isolation in test-project utilities

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

@@ -0,0 +1,10 @@
+/**
+ * Global test isolation for CLI package.
+ * @see packages/core/src/__tests__/setup-test-isolation.ts
+ */
+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;

packages/cli/vitest.config.ts

@@ -14,7 +14,10 @@ export default defineConfig({
   },
   test: {
     include: ["src/**/*.test.ts"],
-    setupFiles: [resolve(__dirname, "../core/src/__test-utils__/vitest-setup.ts")],
+    setupFiles: [
+      "./src/__tests__/setup-test-isolation.ts",
+      resolve(__dirname, "../core/src/__test-utils__/vitest-setup.ts"),
+    ],
     globalSetup: [resolve(__dirname, "../core/src/__test-utils__/vitest-teardown.ts")],
     maxWorkers,
     poolOptions: { threads: { minThreads: 1, maxThreads: maxWorkers }, forks: { minForks: 1, maxForks: maxWorkers } },