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/message-runtime.test.ts

@@ -19,6 +19,7 @@ vi.mock('./config.js', () => ({
   CODEX_MAIN_SERVICE_ID: 'codex-main',
   CODEX_REVIEW_SERVICE_ID: 'codex-review',
   REVIEWER_AGENT_TYPE: 'claude-code',
+  ARBITER_AGENT_TYPE: undefined,
   normalizeServiceId: vi.fn((serviceId: string) => serviceId),
   isClaudeService: vi.fn(() => true),
   isReviewService: vi.fn(() => false),
@@ -556,6 +557,8 @@ describe('createMessageRuntime', () => {
       review_requested_at: '2026-03-30T00:00:00.000Z',
       round_trip_count: 0,
       status: 'review_ready',
+      arbiter_verdict: null,
+      arbiter_requested_at: null,
       created_at: '2026-03-30T00:00:00.000Z',
       updated_at: '2026-03-30T00:00:00.000Z',
     });