fix: Windows compatibility and flaky benchmark test

- Use cmd.exe /c to execute .bat/.cmd on Windows
- Set USERPROFILE alongside HOME for os.UserHomeDir()
- Use setTempDirEnv to set TEMP/TMP on Windows
- Replace chmod-based tests with cross-platform alternatives
- Fix concurrent speedup benchmark with fair comparison
- Add output/ to gitignore

Generated with SWE-Agent.ai

Co-Authored-By: SWE-Agent.ai <noreply@swe-agent.ai>

codeagent-wrapper/internal/app/stdin_mode_test.go

@@ -8,7 +8,7 @@ import (
 func TestRunSingleMode_UseStdin_TargetArgAndTaskText(t *testing.T) {
 	defer resetTestHooks()
 
-	t.Setenv("TMPDIR", t.TempDir())
+	setTempDirEnv(t, t.TempDir())
 	logger, err := NewLogger()
 	if err != nil {
 		t.Fatalf("NewLogger(): %v", err)