feat(FN-834): add per-workflow-step model override UI and support

- Add model provider/model selection fields to WorkflowStepManager component
- Add dropdowns for choosing executor model per workflow step definition
- Wire executor to use per-step model overrides when configured
- Add comprehensive tests for WorkflowStepManager (340 lines)
- Add CSS styles for model selection UI in workflow step editor
- Update AGENTS.md and README.md with model override documentation
- Add changeset for published package bump

packages/dashboard/app/styles.css

@@ -12329,6 +12329,42 @@ html .column.drag-over * {
   margin: 0 auto var(--space-sm);
 }
 
+.wfm-model-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 8px;
+}
+
+.wfm-model-header label {
+  margin-bottom: 0;
+}
+
+.wfm-model-clear-btn {
+  display: inline-flex;
+  align-items: center;
+  gap: 4px;
+  font-size: 11px;
+  color: var(--text-tertiary);
+  background: none;
+  border: none;
+  cursor: pointer;
+  padding: 2px 6px;
+  border-radius: 4px;
+}
+
+.wfm-model-clear-btn:hover {
+  color: var(--text-secondary);
+  background: var(--bg-tertiary);
+}
+
+.wfm-model-hint {
+  display: block;
+  font-size: 11px;
+  color: var(--text-tertiary);
+  margin-bottom: 6px;
+}
+
 .new-task-modal .form-group small {
   display: block;
   margin-top: var(--space-sm);