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.test.ts

@@ -44,6 +44,7 @@ vi.mock('./service-routing.js', () => ({
     chat_jid: 'dc:test',
     owner_service_id: 'claude',
     reviewer_service_id: 'codex-main',
+    arbiter_service_id: null,
     activated_at: null,
     reason: null,
     explicit: false,
@@ -206,6 +207,7 @@ describe('prepareGroupEnvironment codex auth handling', () => {
       chat_jid: 'dc:test',
       owner_service_id: 'codex-review',
       reviewer_service_id: 'codex-main',
+      arbiter_service_id: null,
       activated_at: '2026-03-28T00:00:00.000Z',
       reason: 'claude-429',
       explicit: true,
@@ -310,6 +312,7 @@ describe('prepareGroupEnvironment codex auth handling', () => {
       chat_jid: 'dc:test',
       owner_service_id: 'claude',
       reviewer_service_id: 'codex-main',
+      arbiter_service_id: null,
       activated_at: null,
       reason: null,
       explicit: false,