Add workspace configuration support

internal/core/config/validation.go

@@ -12,6 +12,9 @@ func (c Config) Validate() error {
 	if err := validateScriptorium(c.Scriptorium); err != nil {
 		return err
 	}
+	if err := validateWorkspace(c.Workspace); err != nil {
+		return err
+	}
 	if err := validateDiagnostics(c.Diagnostics); err != nil {
 		return err
 	}
@@ -28,6 +31,17 @@ func validateScriptorium(cfg ScriptoriumConfig) error {
 	return nil
 }
 
+func validateWorkspace(cfg WorkspaceConfig) error {
+	if cfg.Diagnostics.retentionSet {
+		switch cfg.Diagnostics.Retention {
+		case "", diagnostics.RetentionAuto, diagnostics.RetentionAlways, diagnostics.RetentionNever:
+		default:
+			return fmt.Errorf("workspace diagnostics retention %q is not supported", cfg.Diagnostics.Retention)
+		}
+	}
+	return nil
+}
+
 func validateDiagnostics(cfg DiagnosticsConfig) error {
 	if strings.TrimSpace(cfg.WorkDir) == "" {
 		return fmt.Errorf("diagnostics work dir must not be empty")