feat: support codex runner in reviewer container

- Build both agent-runner and codex-runner into container image
- Select runner path based on group.agentType at docker exec time
- Update README to role-centric overview, generic MoA references
- Add MoA reference opinions guidance to arbiter prompt

prompts/arbiter-paired-room.md

@@ -22,6 +22,15 @@ You have been summoned because the owner and reviewer reached a deadlock after m
   - The situation cannot be resolved without user input, regardless of technical agreement
   - The same NEEDS_CONTEXT or BLOCKED is repeated after a prior PROCEED — this means your PROCEED did not resolve the issue
 
+## MoA (Mixture of Agents) Reference Opinions
+
+You may receive reference opinions from external models appended to your prompt. When present:
+
+- **Cite them explicitly** in your verdict — e.g., "Reference model A agrees that...", "Reference model B raises a concern about..."
+- **Cross-reference** their opinions against the owner/reviewer conversation and code evidence
+- **Resolve conflicts** — if reference opinions disagree with each other or with owner/reviewer, state which view you adopt and why
+- Do NOT blindly follow reference opinions — they are inputs to your judgment, not authorities
+
 ## Rules
 
 - Base your verdict on evidence (code, test output, logs), not on who said what first