fix: tighten config and update error handling

internal/config/config.go

@@ -67,6 +67,9 @@ func Validate(cfg domain.Config) error {
 	if strings.TrimSpace(cfg.Gitea.Org) == "" {
 		return errors.New("gitea org is required")
 	}
+	if strings.TrimSpace(cfg.Gitea.Username) == "" {
+		return errors.New("gitea username is required")
+	}
 	if cfg.Gitea.AuthType != AuthPassword && cfg.Gitea.AuthType != AuthToken {
 		return errors.New("authType must be password or token")
 	}

internal/config/config_test.go

@@ -62,6 +62,11 @@ func TestValidateRejectsMissingConnectionFields(t *testing.T) {
 	}
 
 	cfg.Gitea.Org = "skills"
+	if err := Validate(cfg); err == nil {
+		t.Fatal("Validate returned nil error without username")
+	}
+
+	cfg.Gitea.Username = "alice"
 	if err := Validate(cfg); err != nil {
 		t.Fatalf("Validate returned error for valid config: %v", err)
 	}