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/paired-workspace-manager.test.ts

@@ -78,6 +78,8 @@ describe('paired workspace manager', () => {
       round_trip_count: 0,
       review_requested_at: null,
       status: 'active',
+      arbiter_verdict: null,
+      arbiter_requested_at: null,
       created_at: now,
       updated_at: now,
     });
@@ -167,6 +169,8 @@ describe('paired workspace manager', () => {
       round_trip_count: 0,
       review_requested_at: null,
       status: 'active',
+      arbiter_verdict: null,
+      arbiter_requested_at: null,
       created_at: now,
       updated_at: now,
     });
@@ -243,6 +247,8 @@ describe('paired workspace manager', () => {
       round_trip_count: 0,
       review_requested_at: null,
       status: 'active',
+      arbiter_verdict: null,
+      arbiter_requested_at: null,
       created_at: now,
       updated_at: now,
     });
@@ -318,6 +324,8 @@ describe('paired workspace manager', () => {
       round_trip_count: 0,
       review_requested_at: null,
       status: 'active',
+      arbiter_verdict: null,
+      arbiter_requested_at: null,
       created_at: now,
       updated_at: now,
     });
@@ -459,6 +467,8 @@ describe('paired workspace manager', () => {
       round_trip_count: 0,
       review_requested_at: null,
       status: 'active',
+      arbiter_verdict: null,
+      arbiter_requested_at: null,
       created_at: now,
       updated_at: now,
     });
@@ -521,6 +531,8 @@ describe('paired workspace manager', () => {
       round_trip_count: 0,
       review_requested_at: '2026-03-28T00:01:00.000Z',
       status: 'review_ready',
+      arbiter_verdict: null,
+      arbiter_requested_at: null,
       created_at: now,
       updated_at: '2026-03-28T00:01:00.000Z',
     });
@@ -587,6 +599,8 @@ describe('paired workspace manager', () => {
       round_trip_count: 0,
       review_requested_at: null,
       status: 'active',
+      arbiter_verdict: null,
+      arbiter_requested_at: null,
       created_at: now,
       updated_at: now,
     });