feat: implement MAGI 3-agent arbiter system for deadlock resolution

Introduces a third agent role (arbiter) that is summoned on-demand
when owner and reviewer reach a deadlock (same verdict 3+ rounds).

Architecture:
- 3 Discord bots: owner (codex), reviewer (claude), arbiter (claude/codex)
- Arbiter is NOT always-on — only invoked when deadlock detected
- Arbiter renders binding verdict: PROCEED/REVISE/RESET/ESCALATE
- Non-escalate verdicts reset round_trip_count and resume ping-pong
- Backward compatible: ARBITER_AGENT_TYPE unset = existing 2-agent mode

Changes across 13 source files + 7 test files:
- types.ts: PairedRoomRole += 'arbiter', new statuses, ArbiterVerdict type
- config.ts: ARBITER_AGENT_TYPE, ARBITER_SERVICE_ID, ARBITER_DEADLOCK_THRESHOLD
- db.ts: schema migration (arbiter columns in channel_owner + paired_tasks)
- service-routing.ts: arbiter_service_id in lease
- room-role-context.ts: arbiter role detection
- paired-execution-context.ts: deadlock->arbiter, verdict handling
- message-runtime.ts: arbiter turn routing, cursor, sender labeling
- message-agent-executor.ts: arbiter mode, failover
- agent-runner.ts + environment.ts: arbiter container mode
- platform-prompts.ts: arbiter prompt loading

New files:
- prompts/arbiter-paired-room.md: arbiter system prompt
- src/arbiter-context.ts: builds conversation context for arbiter judgment

src/room-role-context.ts

@@ -20,12 +20,20 @@ export function buildRoomRoleContext(
   }
 
   const ownerServiceId = normalizeServiceId(lease.owner_service_id);
+  const arbiterServiceId = lease.arbiter_service_id
+    ? normalizeServiceId(lease.arbiter_service_id)
+    : undefined;
+
+  // Check arbiter role first: if this service matches the arbiter_service_id,
+  // it takes the arbiter role (even if it also matches owner or reviewer)
   const role =
-    ownerServiceId === normalizedServiceId
-      ? 'owner'
-      : reviewerServiceId === normalizedServiceId
-        ? 'reviewer'
-        : null;
+    arbiterServiceId && arbiterServiceId === normalizedServiceId
+      ? 'arbiter'
+      : ownerServiceId === normalizedServiceId
+        ? 'owner'
+        : reviewerServiceId === normalizedServiceId
+          ? 'reviewer'
+          : null;
 
   if (!role) {
     return undefined;
@@ -39,5 +47,6 @@ export function buildRoomRoleContext(
     failoverOwner:
       ownerServiceId === CODEX_REVIEW_SERVICE_ID &&
       reviewerServiceId === CODEX_MAIN_SERVICE_ID,
+    arbiterServiceId,
   };
 }