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

@@ -0,0 +1,29 @@
+import { describe, expect, it } from "vitest";
+import { homedir, tmpdir } from "node:os";
+
+const TEMP_HOME_PREFIX = "fn-test-home-";
+
+describe("test isolation setup", () => {
+  it("process.env.HOME is overridden to a temp directory", () => {
+    const home = process.env.HOME;
+
+    expect(home).toBeDefined();
+    expect(home).toContain(tmpdir());
+    expect(home).toContain(TEMP_HOME_PREFIX);
+  });
+
+  it("homedir() resolves to the temp HOME", () => {
+    const home = homedir();
+
+    expect(home).toContain(tmpdir());
+    expect(home).toContain(TEMP_HOME_PREFIX);
+  });
+
+  it("defaultGlobalDir() resolves under the temp HOME", async () => {
+    const { defaultGlobalDir } = await import("../global-settings.js");
+    const dir = defaultGlobalDir();
+
+    expect(dir).toContain(tmpdir());
+    expect(dir).toMatch(/fn-test-home-.*[\\/]\.fusion$/);
+  });
+});

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

@@ -0,0 +1,16 @@
+/**
+ * Global test isolation: prevents tests from writing to the real ~/.fusion/ directory.
+ *
+ * Vitest runs setupFiles in each worker thread. By overriding process.env.HOME
+ * to a temp directory, all calls to homedir() (and derived paths like ~/.fusion)
+ * resolve to isolated temp locations instead of the user's real home directory.
+ *
+ * This protects against tests accidentally creating projects, databases, or
+ * settings files in the production ~/.fusion/ directory.
+ */
+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/core/src/__tests__/test-project.ts

@@ -55,6 +55,10 @@ function splitSettings(settings?: Partial<Settings>): {
 }
 
 /**
+ * NOTE: Global test isolation overrides HOME in vitest setupFiles,
+ * so homedir()/resolveGlobalDir() always resolve to temp directories.
+ * This file provides per-fixture isolation on top of that safety net.
+ *
  * Create an isolated temporary test project with a real TaskStore + SQLite DB.
  */
 export async function createTestProject(