test: extraer resolveWorkDir y tests unitarios de aislamiento

Extraer la logica de resolucion de working_dir a una funcion
resolveWorkDir() separada para hacerla testeable. Tres tests cubren:
- WorkingDir vacio → crea tmpdir con prefijo claude-agent-*
- WorkingDir configurado → crea el directorio y lo usa
- WorkingDir ya existente → lo usa sin error

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shell/llm/claudecode_test.go

@@ -5,6 +5,9 @@ import (
 	"errors"
 	"io"
 	"log/slog"
+	"os"
+	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -318,6 +321,56 @@ func TestFilterEnv_PrefixSafety(t *testing.T) {
 	}
 }
 
+// ── resolveWorkDir ──────────────────────────────────────────────────────
+
+func TestResolveWorkDir_EmptyCreatesTempDir(t *testing.T) {
+	dir := resolveWorkDir("", discardLog)
+	if dir == "" {
+		t.Fatal("expected a temp directory, got empty string")
+	}
+	defer os.RemoveAll(dir)
+
+	if !strings.Contains(dir, "claude-agent-") {
+		t.Errorf("temp dir %q should contain 'claude-agent-' prefix", dir)
+	}
+
+	info, err := os.Stat(dir)
+	if err != nil {
+		t.Fatalf("temp dir should exist: %v", err)
+	}
+	if !info.IsDir() {
+		t.Error("temp dir should be a directory")
+	}
+}
+
+func TestResolveWorkDir_ConfiguredValueUsed(t *testing.T) {
+	want := filepath.Join(t.TempDir(), "custom-workdir")
+
+	got := resolveWorkDir(want, discardLog)
+
+	if got != want {
+		t.Errorf("got %q, want %q", got, want)
+	}
+
+	info, err := os.Stat(got)
+	if err != nil {
+		t.Fatalf("configured dir should be created: %v", err)
+	}
+	if !info.IsDir() {
+		t.Error("configured dir should be a directory")
+	}
+}
+
+func TestResolveWorkDir_ConfiguredAlreadyExists(t *testing.T) {
+	want := t.TempDir() // already exists
+
+	got := resolveWorkDir(want, discardLog)
+
+	if got != want {
+		t.Errorf("got %q, want %q", got, want)
+	}
+}
+
 // ── helpers ──────────────────────────────────────────────────────────────
 
 func contains(s, substr string) bool {