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;