fix(home): fail loud instead of falling back to os.tmpdir in getUserHome

src/__tests__/home.test.ts

@@ -70,16 +70,16 @@ describe('getUserHome', () => {
     expect(getUserHome()).toBe(os.homedir());
   });
 
-  it('falls back to os.tmpdir when even os.homedir is unresolvable', () => {
+  it('throws instead of returning a shared/relative dir when home is unresolvable', () => {
     delete process.env.HOME;
     delete process.env.USERPROFILE;
     // os.homedir() is documented to return '' when the home cannot be resolved.
+    // Falling back to os.tmpdir() (world-writable) or '' (cwd-relative) would be a
+    // credential-exposure / code-execution vector, so getUserHome() must throw.
     const spy = vi.spyOn(os, 'homedir').mockReturnValue('');
 
     try {
-      const home = getUserHome();
-      expect(home).toBe(os.tmpdir());
-      expect(home).not.toBe('');
+      expect(() => getUserHome()).toThrow(/user home directory/);
     } finally {
       spy.mockRestore();
     }

src/utils/home.ts

@@ -4,16 +4,26 @@ import os from 'node:os';
  * Resolve the current user's home directory across supported platforms.
  *
  * `HOME` is normally present on Unix-like systems, while a regular Windows
- * PowerShell session commonly exposes only `USERPROFILE`. `os.homedir()` is
- * the platform-aware fallback, and `os.tmpdir()` is a last resort: `os.homedir()`
- * is documented to return `''` when the home cannot be resolved (e.g. a passwd-less
- * uid or `env -i`), and callers join this onto `.teamai/...`, so an empty result
- * would silently yield a cwd-relative path. Guaranteeing a non-empty absolute path
- * keeps every derived path absolute.
+ * PowerShell session commonly exposes only `USERPROFILE`. `os.homedir()` is the
+ * platform-aware fallback.
+ *
+ * If none of these resolves (os.homedir() is documented to return '' for a
+ * passwd-less uid or under `env -i`), we throw rather than fall back to a
+ * shared/relative directory. Callers join this onto `.teamai/...` to write
+ * credentials and to resolve executables, so a shared location like os.tmpdir()
+ * would be a code-execution / credential-exposure vector, and an empty string
+ * would silently produce a cwd-relative path. Failing loudly is the safe choice —
+ * on any real machine at least one of the three is always set.
  */
 export function getUserHome(): string {
-  return process.env.HOME?.trim()
+  const home = process.env.HOME?.trim()
     || process.env.USERPROFILE?.trim()
-    || os.homedir()
-    || os.tmpdir();
+    || os.homedir();
+  if (!home) {
+    throw new Error(
+      'Unable to determine the user home directory: none of HOME, USERPROFILE, ' +
+      'or os.homedir() is available. Set HOME explicitly before running teamai.',
+    );
+  }
+  return home;
 }