feat(user-interaction): declare a plan-review presentation intent on questions

A question may now carry `intent`, a tagged declaration that it IS a decision
of a known shape, so a UI that recognises the tag can present it as such
instead of as a generic option list. The one member is
`{ kind: 'plan-review', approve }`, which plan-mode sets on the exit_plan_mode
review.

An intent shapes presentation only: a UI honouring it answers with the same
option labels a generic UI would send, so the tool reads one answer shape
either way, and a UI that does not know the tag renders the generic flow.
`approve` names the affirmative option rather than relying on option order;
since no type can tie that label to the question's own option list, `ask()`
rejects a mismatch as BAD_INTENT, and the wire schema rejects an unknown tag
outright rather than silently rendering generic.

plan-mode also stops reporting a dismissed review as "the user cancelled
ask_user_question" — a tool it never called. A dismissal now tells the model
the user took the turn back to speak, and to stay in plan mode and wait; every
other ask failure keeps its own message.

packages/host/apiproxy/src/api/events.schema.ts

@@ -23,6 +23,11 @@ export const askUserQuestionItemSchema = z.object({
   detail: z.string().optional(),
   options: z.array(z.object({ label: z.string(), description: z.string().optional() })).optional(),
   multiSelect: z.boolean().optional(),
+  // Presentation intent: a tagged union on the wire, so an unknown tag is a
+  // rejected frame rather than a silently generic render.
+  intent: z.discriminatedUnion('kind', [
+    z.object({ kind: z.literal('plan-review'), approve: z.string() }),
+  ]).optional(),
 }) satisfies z.ZodType<Wire<AskUserQuestionItem>>
 
 /** Unified message envelope carried by transient queue frames. */

packages/host/apiproxy/tests/rpc-schemas.spec.ts

@@ -399,6 +399,17 @@ describe('events frame schemas', () => {
     expect(() => muxFrameSchema.parse({ type: 'question/requested', sessionId: 's', questions: [] })).toThrow()
   })
 
+  it('carries a question presentation intent through, and rejects an unknown one', () => {
+    const intent = { kind: 'plan-review', approve: 'Approve' }
+    expect(askUserQuestionItemSchema.parse({
+      id: 'plan-review', question: 'Approve?', detail: '# Plan', options: [{ label: 'Approve' }], intent,
+    }).intent).toEqual(intent)
+    // An unrecognised tag is a rejected frame, not a silently generic render.
+    for (const invalid of [{ kind: 'plan-review' }, { kind: 'poll', approve: 'Approve' }, { approve: 'Approve' }]) {
+      expect(() => askUserQuestionItemSchema.parse({ id: 'q', question: 'Q?', intent: invalid })).toThrow()
+    }
+  })
+
   it('rejects a queue snapshot with malformed items', () => {
     expect(() => muxFrameSchema.parse({ type: 'session/queue', sessionId: 's', items: 'x' })).toThrow()
     expect(() => muxFrameSchema.parse({ type: 'session/queue', sessionId: 's', items: [{ id: '', message: {} }] })).toThrow()