feat(dashboard): make built-in workflows read-only in the node editor

Built-in (builtin:*) workflows are read-only at the store layer; reflect
that in the editor so users aren't misled into editing changes that can't
save. For a built-in: the node palette is disabled, the inspector fields are
wrapped in a disabled fieldset, Save/Delete are replaced by a "Read-only
built-in" label, and a "Duplicate to edit" action clones it into an editable
user workflow. handleSave/handleDeleteWorkflow also early-return for built-ins
as defense-in-depth.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/dashboard/app/components/WorkflowNodeEditor.css

@@ -159,9 +159,26 @@
 
 .wf-editor-actions {
   display: flex;
+  align-items: center;
   gap: var(--space-xs);
 }
 
+.wf-editor-readonly-note {
+  font-size: 0.75rem;
+  color: var(--text-tertiary);
+  text-transform: uppercase;
+  letter-spacing: 0.04em;
+}
+
+/* Neutralize the fieldset wrapper so it only gates interactivity, not layout. */
+.wf-inspector-fields {
+  display: contents;
+  border: none;
+  margin: 0;
+  padding: 0;
+  min-inline-size: 0;
+}
+
 .wf-editor-save {
   background: var(--todo);
   border-color: var(--todo);