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/arbiter-context.ts

@@ -0,0 +1,29 @@
+import { getRecentChatMessages } from './db.js';
+import { formatMessages } from './router.js';
+
+export function buildArbiterContextPrompt(args: {
+  chatJid: string;
+  taskId: string;
+  roundTripCount: number;
+  timezone: string;
+  recentTurnLimit?: number;
+}): string {
+  const { chatJid, taskId, roundTripCount, timezone, recentTurnLimit = 20 } = args;
+
+  const recentMessages = getRecentChatMessages(chatJid, recentTurnLimit);
+  const conversationContext = formatMessages(recentMessages, timezone);
+
+  return [
+    `<arbiter-context>`,
+    `<task-id>${taskId}</task-id>`,
+    `<round-trips>${roundTripCount}</round-trips>`,
+    `<reason>Deadlock detected: owner and reviewer exchanged ${roundTripCount} rounds without resolution</reason>`,
+    `</arbiter-context>`,
+    ``,
+    `<conversation-history>`,
+    conversationContext,
+    `</conversation-history>`,
+    ``,
+    `Review the conversation above and render your verdict.`,
+  ].join('\n');
+}