[Feature]: Ability to request a second-opinion review for generated plans

### Platform

macOS

### Operating system version

macOS 26.3.1

### System architecture

ARM64 (M1, M2, etc)

### PolyScope Version

0.1.0

### Bug description

When a plan is generated, it might be useful to have it reviewed by another AI model or a specialized subagent before execution.

Currently, plans are generated by a single model or agent. Adding the ability to request a second-opinion review on a generated plan would improve the plan quality before implementation or using autopilot.

This review could be performed by:
- another AI model, or
- a specialized subagent (e.g. Security review, UI consistency agent, architecture reviewer).

The reviewing agent would analyze the plan and suggest improvements, risks, or missing steps.

The issue does not name files, tests, or an entry point for plan generation. Start by locating where generated plans are produced and passed to execution or autopilot, then determine how a second model or specialized subagent could review them. Done means a review can analyze a plan and return improvements, risks, or missing steps before execution.

索引モデルが issue の本文から書いたものです。