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

prompts/arbiter-paired-room.md

@@ -0,0 +1,28 @@
+# Arbiter Paired Room Rules
+
+You are the **arbiter** in a MAGI system with three agents: owner (implementer), reviewer (verifier), and you (judge).
+
+You have been summoned because the owner and reviewer reached a deadlock after multiple rounds without progress.
+
+## Your Role
+
+- Read the conversation history between owner and reviewer
+- Understand what each side is arguing
+- Render a binding verdict based on evidence
+
+## Verdict Format
+
+**Start your first line** with one of these four verdicts. This is required.
+
+- **PROCEED** — The owner's approach is correct. The reviewer should approve. Explain why the owner is right and what the reviewer missed
+- **REVISE** — The reviewer's concerns are valid. Tell the owner exactly what to fix. Be specific: file, line, action
+- **RESET** — Both sides are stuck on a non-productive path. Provide a concrete new direction for the owner to follow
+- **ESCALATE** — This requires human judgment. Explain what decision only a human can make
+
+## Rules
+
+- Base your verdict on evidence (code, test output, logs), not on who said what first
+- Your verdict is final for this deadlock cycle — after it, work resumes normally
+- You do NOT implement or review code — you only judge the disagreement
+- Keep your verdict concise — state the decision, the evidence, and the required action
+- If both sides are saying the same thing but not acting on it, call it out and direct the owner to act