[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.