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.test.ts

@@ -10,6 +10,7 @@ describe('buildRoomRoleContext', () => {
           chat_jid: 'group@test',
           owner_service_id: 'claude',
           reviewer_service_id: 'codex-main',
+          arbiter_service_id: null,
           activated_at: null,
           reason: null,
           explicit: false,
@@ -22,6 +23,7 @@ describe('buildRoomRoleContext', () => {
       ownerServiceId: 'claude',
       reviewerServiceId: 'codex-main',
       failoverOwner: false,
+      arbiterServiceId: undefined,
     });
   });
 
@@ -32,6 +34,7 @@ describe('buildRoomRoleContext', () => {
           chat_jid: 'group@test',
           owner_service_id: 'codex-review',
           reviewer_service_id: 'codex-main',
+          arbiter_service_id: null,
           activated_at: '2026-03-28T10:00:00.000Z',
           reason: 'claude-429',
           explicit: true,
@@ -44,6 +47,31 @@ describe('buildRoomRoleContext', () => {
       ownerServiceId: 'codex-review',
       reviewerServiceId: 'codex-main',
       failoverOwner: true,
+      arbiterServiceId: undefined,
+    });
+  });
+
+  it('returns arbiter context when service matches arbiter_service_id', () => {
+    expect(
+      buildRoomRoleContext(
+        {
+          chat_jid: 'group@test',
+          owner_service_id: 'codex-main',
+          reviewer_service_id: 'claude',
+          arbiter_service_id: 'codex-review',
+          activated_at: null,
+          reason: null,
+          explicit: false,
+        },
+        'codex-review',
+      ),
+    ).toEqual({
+      serviceId: 'codex-review',
+      role: 'arbiter',
+      ownerServiceId: 'codex-main',
+      reviewerServiceId: 'claude',
+      failoverOwner: false,
+      arbiterServiceId: 'codex-review',
     });
   });
 
@@ -54,6 +82,7 @@ describe('buildRoomRoleContext', () => {
           chat_jid: 'solo@test',
           owner_service_id: 'codex-main',
           reviewer_service_id: null,
+          arbiter_service_id: null,
           activated_at: null,
           reason: null,
           explicit: false,