fix(backend): echo Pi's RPC id so reviewer gates unblock after answer

ctx.ui.input in `pi --mode rpc` correlates extension_ui_response on its own
top-level RPC id (crypto.randomUUID), not the descriptor id the gate carries
in `title`. SessionBridge replied with the descriptor id, so Pi silently
dropped the response and the model never resumed — every reviewer widget hung
after the human answered.

SessionBridge now stores Pi's top-level m.id (pendingPiId) and replies
extension_ui_response{ id: pendingPiId, value: <uiResponse> }; value still
carries the descriptor id so the gate's internal resp.id === descriptor.id
check still holds.

The fake-pi double had masked the bug by forcing m.id == descriptor.id; it now
mirrors real Pi (distinct randomUUID, correlate on it, drop unknown ids), with
a negative regression test. SKILL.md Phase 1 also now steers multi-answer
disambiguation to reviewer_decide (multiselect).

Tests: backend 67/67, tsc clean, fake-pi contract 2/2.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/test/e2e-f1.test.ts

@@ -7,7 +7,28 @@ import { loadConfig } from "../src/config.js";
 const FAKE = path.resolve("../harness/tests/fake_pi/fake_pi_rpc.mjs");
 const SCRIPT = path.resolve("../harness/tests/fake_pi/scripts/f1_disambiguation.json");
 
-test("loop F1: crea sessione → SSE riceve il widget → risponde → 204", async () => {
+// Legge dallo stream SSE finche' `predicate(acc)` e' vero o scade il timeout.
+async function readUntil(
+  reader: ReadableStreamDefaultReader<Uint8Array>,
+  predicate: (acc: string) => boolean,
+  timeoutMs: number,
+): Promise<string> {
+  const dec = new TextDecoder();
+  let acc = "";
+  const deadline = Date.now() + timeoutMs;
+  while (Date.now() < deadline) {
+    const res: any = await Promise.race([
+      reader.read(),
+      new Promise((r) => setTimeout(() => r({ timeout: true }), deadline - Date.now())),
+    ]);
+    if (res.timeout || res.done) break;
+    acc += dec.decode(res.value);
+    if (predicate(acc)) return acc;
+  }
+  return acc;
+}
+
+test("loop F1: crea sessione → SSE riceve il widget → risponde → il modello riparte (follow-up)", async () => {
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     thtRunner: {
       ollamaEnsure: async () => ({ ok: true }),
@@ -30,15 +51,13 @@ test("loop F1: crea sessione → SSE riceve il widget → risponde → 204", asy
   // 2. Small delay to let fake-pi process the prompt and fill pendingWidget
   await new Promise((r) => setTimeout(r, 50));
 
-  // 3. SSE: read the first event (pendingWidget re-emit via subscribe)
+  // 3. SSE: the pending widget is re-emitted on subscribe
   const es = await fetch(`${base}/sessions/s1/events`);
   const reader = es.body!.getReader();
-  const chunk = await reader.read();
-  const text = new TextDecoder().decode(chunk.value);
-  expect(text).toContain("ui_request");
-  await reader.cancel();
+  const first = await readUntil(reader, (t) => t.includes("ui_request"), 2000);
+  expect(first).toContain("ui_request");
 
-  // 4. POST response — widget id is "u1" from f1_disambiguation.json
+  // 4. POST response — widget id is "u1" from f1_disambiguation.json (id INTERNO del descriptor)
   const resp = await fetch(`${base}/sessions/s1/response`, {
     method: "POST",
     headers: { "content-type": "application/json" },
@@ -46,5 +65,12 @@ test("loop F1: crea sessione → SSE riceve il widget → risponde → 204", asy
   });
   expect(resp.status).toBe(204);
 
+  // 5. Prova del fix: la risposta deve essere correlata sull'id RPC di Pi, cosi' ctx.ui.input
+  //    si risolve e il modello produce il follow-up. Col bug, la risposta veniva scartata e
+  //    nessun follow-up arrivava ("stuck senza output").
+  const after = await readUntil(reader, (t) => t.includes("Procedo."), 2000);
+  expect(after).toContain("Procedo.");
+  await reader.cancel();
+
   await app.close();
 }, 10000);