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>

harness/tests/fake_pi/scripts/f1_disambiguation.json

@@ -6,6 +6,9 @@
         "options": [ {"id":"a","label":"interpretazione A"}, {"id":"b","label":"interpretazione B"} ],
         "reserved": ["back","exit","other"] } }
   ],
-  "on_response": { "u1": [ { "type": "agent_end" } ] },
+  "on_response": { "u1": [
+    { "type": "message_update", "assistantMessageEvent": { "type": "text_delta", "contentIndex": 0, "delta": "Procedo." } },
+    { "type": "agent_end" }
+  ] },
   "available_models": [ {"provider":"zai","id":"glm-5.2"} ]
 }