fix(config): keep empty agents.defaults.model in saved config

pkg/config/config.go

@@ -172,7 +172,7 @@ type AgentDefaults struct {
 	RestrictToWorkspace bool     `json:"restrict_to_workspace"           env:"PICOCLAW_AGENTS_DEFAULTS_RESTRICT_TO_WORKSPACE"`
 	Provider            string   `json:"provider"                        env:"PICOCLAW_AGENTS_DEFAULTS_PROVIDER"`
 	ModelName           string   `json:"model_name,omitempty"            env:"PICOCLAW_AGENTS_DEFAULTS_MODEL_NAME"`
-	Model               string   `json:"model,omitempty"                 env:"PICOCLAW_AGENTS_DEFAULTS_MODEL"` // Deprecated: use model_name instead
+	Model               string   `json:"model"                           env:"PICOCLAW_AGENTS_DEFAULTS_MODEL"` // Deprecated: use model_name instead
 	ModelFallbacks      []string `json:"model_fallbacks,omitempty"`
 	ImageModel          string   `json:"image_model,omitempty"           env:"PICOCLAW_AGENTS_DEFAULTS_IMAGE_MODEL"`
 	ImageModelFallbacks []string `json:"image_model_fallbacks,omitempty"`

pkg/config/config_test.go

@@ -5,6 +5,7 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"strings"
 	"testing"
 )
 
@@ -324,6 +325,25 @@ func TestSaveConfig_FilePermissions(t *testing.T) {
 	}
 }
 
+func TestSaveConfig_IncludesEmptyLegacyModelField(t *testing.T) {
+	tmpDir := t.TempDir()
+	path := filepath.Join(tmpDir, "config.json")
+
+	cfg := DefaultConfig()
+	if err := SaveConfig(path, cfg); err != nil {
+		t.Fatalf("SaveConfig failed: %v", err)
+	}
+
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("ReadFile failed: %v", err)
+	}
+
+	if !strings.Contains(string(data), `"model": ""`) {
+		t.Fatalf("saved config should include empty legacy model field, got: %s", string(data))
+	}
+}
+
 // TestConfig_Complete verifies all config fields are set
 func TestConfig_Complete(t *testing.T) {
 	cfg := DefaultConfig()