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/types.ts

@@ -23,14 +23,18 @@ export type VisiblePhase = 'silent' | 'progress' | 'final';
 
 export type AgentVisibility = 'public' | 'silent';
 
-export type PairedRoomRole = 'owner' | 'reviewer';
+export type PairedRoomRole = 'owner' | 'reviewer' | 'arbiter';
 
 export type PairedTaskStatus =
   | 'active'
   | 'review_ready'
   | 'in_review'
   | 'merge_ready'
-  | 'completed';
+  | 'completed'
+  | 'arbiter_requested'
+  | 'in_arbitration';
+
+export type ArbiterVerdict = 'proceed' | 'revise' | 'reset' | 'escalate';
 
 export type PairedWorkspaceRole = 'owner' | 'reviewer';
 
@@ -42,6 +46,7 @@ export interface RoomRoleContext {
   ownerServiceId: string;
   reviewerServiceId: string;
   failoverOwner: boolean;
+  arbiterServiceId?: string;
 }
 
 export interface PairedProject {
@@ -64,6 +69,8 @@ export interface PairedTask {
   review_requested_at: string | null;
   round_trip_count: number;
   status: PairedTaskStatus;
+  arbiter_verdict: string | null;
+  arbiter_requested_at: string | null;
   created_at: string;
   updated_at: string;
 }