FN-6165: fix builtin workflow prompt expansion

Keep builtin workflow prompts expandable while preserving read-only behavior.

- move the builtin-disabled fieldset so the prompt expand control stays clickable
- mark inline and fullscreen builtin prompt textareas read-only instead of disabling them
- add coverage for desktop and mobile builtin prompt expansion behavior in WorkflowNodeEditor

Files changed:
 .../app/components/WorkflowNodeEditor.tsx          |  7 +++
 .../__tests__/WorkflowNodeEditor.test.tsx          | 61 ++++++++++++++++++++++
 2 files changed, 68 insertions(+)

Fusion-Task-Id: FN-6165

Fusion-Task-Lineage: 02b1dd9f-6034-46a4-8790-75f1170ddb84

packages/dashboard/app/components/WorkflowNodeEditor.tsx

@@ -2148,6 +2148,7 @@ function InnerEditor({
               <textarea
                 rows={undefined}
                 value={selectedNodePromptValue}
+                readOnly={isBuiltin}
                 onChange={(e) => updateSelectedData({ config: { prompt: e.target.value } })}
                 autoFocus
               />
@@ -3176,6 +3177,7 @@ function InnerEditor({
                   onChange={(e) => updateSelectedData({ label: e.target.value })}
                 />
               </label>
+              </fieldset>
 
               {selectedNode.data.kind === "prompt" || selectedNode.data.kind === "gate" ? (
                 <div className="wf-prompt-editor">
@@ -3184,9 +3186,13 @@ function InnerEditor({
                     <textarea
                       rows={5}
                       value={selectedNodePromptValue}
+                      readOnly={isBuiltin}
                       onChange={(e) => updateSelectedData({ config: { prompt: e.target.value } })}
                     />
                   </label>
+                  {/* Expand button is outside <fieldset disabled={isBuiltin}> so it remains
+                      clickable for builtin workflows. Root cause: HTML spec disables all
+                      descendant buttons inside a disabled fieldset, including type="button". */}
                   <button
                     type="button"
                     className="btn btn-sm wf-prompt-expand-btn wf-prompt-expand-btn--inline"
@@ -3199,6 +3205,7 @@ function InnerEditor({
                 </div>
               ) : null}
 
+              <fieldset className="wf-inspector-fields" disabled={isBuiltin}>
               {selectedNode.data.kind === "prompt" ? (
                 <>
                   <label className="wf-field">

packages/dashboard/app/components/__tests__/WorkflowNodeEditor.test.tsx

@@ -189,6 +189,15 @@ function builtinPrDef(): WorkflowDefinition {
   };
 }
 
+async function selectBuiltinExecutePromptNode() {
+  await screen.findByTestId("wf-readonly-banner");
+  const promptNodes = await screen.findAllByTestId("wf-node-prompt");
+  const executeNode = promptNodes.find((node) => within(node).queryByText("Execute"));
+  expect(executeNode).toBeTruthy();
+  fireEvent.click(executeNode!);
+  return executeNode!;
+}
+
 function edgeRenderableAssertion(definition: WorkflowDefinition) {
   const flow = irToFlow(definition);
   const nodeIds = new Set(flow.nodes.map((node) => node.id));
@@ -643,6 +652,58 @@ describe("WorkflowNodeEditor", () => {
     expect(getPromptFullscreenOverlay()).toBeNull();
   });
 
+  it("shows a non-disabled expand button and read-only prompt for builtin workflow prompt nodes", async () => {
+    vi.mocked(fetchWorkflows).mockResolvedValue([builtinDef()]);
+
+    render(<WorkflowNodeEditor isOpen onClose={() => {}} addToast={() => {}} />);
+
+    await selectBuiltinExecutePromptNode();
+    const inspector = await screen.findByTestId("wf-node-inspector");
+    expect(within(inspector).getByLabelText("Prompt")).toHaveAttribute("readonly");
+
+    const expand = within(inspector).getByRole("button", { name: "Expand prompt editor" });
+    expect(expand).toBeInTheDocument();
+    expect(expand).not.toBeDisabled();
+  });
+
+  it("opens and collapses the fullscreen prompt editor for builtin workflows", async () => {
+    vi.mocked(fetchWorkflows).mockResolvedValue([builtinDef()]);
+
+    render(<WorkflowNodeEditor isOpen onClose={() => {}} addToast={() => {}} />);
+
+    await selectBuiltinExecutePromptNode();
+    fireEvent.click(await screen.findByRole("button", { name: "Expand prompt editor" }));
+
+    const fullscreenPromptEditor = getPromptFullscreenOverlay();
+    expect(fullscreenPromptEditor).toBeInTheDocument();
+    expect(fullscreenPromptEditor).toHaveClass("wf-prompt-editor--fullscreen");
+    expect(getPromptFullscreenTextarea()).toHaveAttribute("readonly");
+
+    fireEvent.click(within(fullscreenPromptEditor!).getByRole("button", { name: "Collapse prompt editor" }));
+
+    expect(getPromptFullscreenOverlay()).toBeNull();
+  });
+
+  it("opens and collapses the fullscreen prompt editor for builtin workflows on mobile", async () => {
+    mockWorkflowEditorViewport("mobile");
+    vi.mocked(fetchWorkflows).mockResolvedValue([builtinDef()]);
+
+    render(<WorkflowNodeEditor isOpen onClose={() => {}} addToast={() => {}} />);
+
+    fireEvent.click(await screen.findByRole("button", { name: "Default coding workflow" }));
+    await selectBuiltinExecutePromptNode();
+    const inspector = await screen.findByTestId("wf-node-inspector");
+    fireEvent.click(within(inspector).getByRole("button", { name: "Expand prompt editor" }));
+
+    const fullscreenPromptEditor = getPromptFullscreenOverlay();
+    expect(fullscreenPromptEditor).toBeInTheDocument();
+    expect(fullscreenPromptEditor).toHaveClass("wf-prompt-editor--fullscreen");
+
+    fireEvent.click(within(fullscreenPromptEditor!).getByRole("button", { name: "Collapse prompt editor" }));
+
+    expect(getPromptFullscreenOverlay()).toBeNull();
+  });
+
   it("persists mobile fullscreen prompt edits back to the inline textarea", async () => {
     mockWorkflowEditorViewport("mobile");
     vi.mocked(fetchWorkflows).mockResolvedValue([v2Def()]);