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/agent-runner-environment.ts

@@ -23,6 +23,7 @@ import {
   resolveServiceTaskSessionsPath,
 } from './group-folder.js';
 import {
+  readArbiterPrompt,
   readPairedRoomPrompt,
   readPlatformPrompt,
 } from './platform-prompts.js';
@@ -527,8 +528,9 @@ export function prepareContainerSessionEnvironment(args: {
   chatJid: string;
   isMain: boolean;
   memoryBriefing?: string;
+  role?: 'reviewer' | 'arbiter';
 }): void {
-  const { sessionDir, chatJid, isMain, memoryBriefing } = args;
+  const { sessionDir, chatJid, isMain, memoryBriefing, role = 'reviewer' } = args;
   const projectRoot = process.cwd();
 
   fs.mkdirSync(sessionDir, { recursive: true });
@@ -541,10 +543,12 @@ export function prepareContainerSessionEnvironment(args: {
   ];
   syncDirectoryEntries(skillSources, path.join(sessionDir, 'skills'));
 
-  // Build CLAUDE.md with reviewer-appropriate prompts
+  // Build CLAUDE.md with role-appropriate prompts (reviewer or arbiter)
   const claudePlatformPrompt = readPlatformPrompt('claude-code', projectRoot);
   const claudePairedRoomPrompt = isPairedRoomJid(chatJid)
-    ? readPairedRoomPrompt('claude-code', projectRoot)
+    ? (role === 'arbiter'
+        ? readArbiterPrompt(projectRoot)
+        : readPairedRoomPrompt('claude-code', projectRoot))
     : undefined;
   const globalDir = path.join(GROUPS_DIR, 'global');
   const globalClaudeMdPath = path.join(globalDir, 'CLAUDE.md');