fix: tighten config and update error handling

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)
 	}