refactor(custom-chips): move the AI entry point out of OSS into an overlay slot

The 'Create with AI' button referenced the Pro AI agent (hardcoded
prompt, agent event) inside the anonymous OSS dialog — chat/agent logic
must live in the velxio-prod overlay, not here. CustomChipDialog now
just exposes a generic `data-velxio-slot="custom-chip-actions"`
extension point (empty in OSS) and hangs its close handler on the slot
element so the overlay can dismiss the dialog after acting. The button
itself, its prompt and the Pro entitlement gate move to the overlay.

frontend/src/components/customChips/CustomChipDialog.tsx

@@ -283,23 +283,18 @@ export const CustomChipDialog = ({ initial, onClose, onSave }: CustomChipDialogP
               {compiling ? t('editor.customChip.compiling') : t('editor.customChip.compile')}
             </button>
           )}
-          <button
-            style={cancelBtn}
-            title="Describe the chip and the AI agent writes, compiles and programs it"
-            onClick={() => {
-              // Pro AI agent hook — no-op when no overlay listener is mounted.
-              window.dispatchEvent(new CustomEvent('velxio:agent-prompt', {
-                detail: {
-                  prompt:
-                    'Crea un custom chip que ... (describe qué debe hacer; ' +
-                    'el agente escribe el C, lo compila y lo programa en el canvas)',
-                },
-              }));
-              onClose();
+          {/* Extension point for the velxio-prod overlay (e.g. a "Create with
+              AI" button). Empty in OSS. The overlay reads `velxioCloseDialog`
+              off this element to dismiss the dialog after it acts. */}
+          <div
+            data-velxio-slot="custom-chip-actions"
+            style={{ display: 'contents' }}
+            ref={(el) => {
+              if (el) {
+                (el as unknown as { velxioCloseDialog?: () => void }).velxioCloseDialog = onClose;
+              }
             }}
-          >
-            ✨ Create with AI
-          </button>
+          />
           <div style={{ flex: 1 }} />
           <button style={cancelBtn} onClick={onClose}>{t('editor.customChip.cancel')}</button>
           <button style={canSave ? saveBtn : saveBtnDisabled} disabled={!canSave} onClick={doSave}>