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/platform-prompts.ts

@@ -15,6 +15,8 @@ const PAIRED_ROOM_PROMPT_FILES: Record<AgentType, string> = {
   codex: 'claude-paired-room.md',
 };
 
+const ARBITER_PROMPT_FILE = 'arbiter-paired-room.md';
+
 export function getPlatformPromptsDir(projectRoot = process.cwd()): string {
   return path.join(projectRoot, 'prompts');
 }
@@ -60,3 +62,22 @@ export function readPairedRoomPrompt(
   const prompt = fs.readFileSync(promptPath, 'utf-8').trim();
   return prompt || undefined;
 }
+
+export function getArbiterPromptPath(
+  projectRoot = process.cwd(),
+): string {
+  return path.join(
+    getPlatformPromptsDir(projectRoot),
+    ARBITER_PROMPT_FILE,
+  );
+}
+
+export function readArbiterPrompt(
+  projectRoot = process.cwd(),
+): string | undefined {
+  const promptPath = getArbiterPromptPath(projectRoot);
+  if (!fs.existsSync(promptPath)) return undefined;
+
+  const prompt = fs.readFileSync(promptPath, 'utf-8').trim();
+  return prompt || undefined;
+}