fix(coding-agent): set executionMode: sequential on question example tool

The question extension tool blocks on ctx.ui.custom() but omitted
executionMode, so multiple question calls in one assistant turn ran
under the default parallel mode and raced on the editor slot. Only the
last question rendered; earlier calls hung pending forever.

Set executionMode: "sequential" so the agent loop serializes the
batch. Wrappers already propagate the field to the core.

closes #6189

packages/coding-agent/examples/extensions/question.ts

@@ -47,6 +47,7 @@ export default function question(pi: ExtensionAPI) {
 		label: "Question",
 		description: "Ask the user a question and let them pick from options. Use when you need user input to proceed.",
 		parameters: QuestionParams,
+		executionMode: "sequential",
 
 		async execute(_toolCallId, params, _signal, _onUpdate, ctx) {
 			if (ctx.mode !== "tui") {