test(hooks): fix test 5 Windows-compat — PATH=nodeDir not PATH=''

Previous test 5 stripped PATH entirely, which kills node.exe spawn resolution
on Windows (CreateProcess needs PATH to find node). Changed to set PATH to
node's own directory only — node spawns fine, git is not in node-dir → ENOENT
→ hook fail-opens per spec §4.5.

All 5 tests now pass cross-platform.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tools/subagent-prompt-prefix.test.mjs

@@ -82,11 +82,15 @@ test('hook fail-open on malformed stdin', () => {
 });
 
 test('hook fail-open when git not available', () => {
+  // Cross-platform: set PATH to node's own directory only — node spawns OK,
+  // git not in node's bin dir → ENOENT → hook fail-opens per spec §4.5.
+  // (Previous approach `PATH: ''` kills node.exe spawn resolution on Windows.)
+  const nodeDir = dirname(process.execPath);
   const result = spawnSync('node', [HOOK_PATH], {
     input: JSON.stringify({ tool_name: 'Task', tool_input: { prompt: 'x' } }),
     encoding: 'utf8',
     timeout: 5000,
-    env: { ...process.env, PATH: '' }, // strip PATH → git not found
+    env: { PATH: nodeDir, PATHEXT: process.env.PATHEXT || '.EXE' },
   });
   assert.equal(result.status, 0, `should not crash when git missing; stderr: ${result.stderr}`);
 });