fix(plan): render use_cases section in markdown output

[hechibing]

Summary

• add missing ## Use Cases markdown section in PlanHandler output
• render each use_cases entry as a bullet item
• add regression tests for present/absent use_cases

Why

PlanHandler parsed use_cases and included it in metadata, but omitted it from human-readable markdown, making plans incomplete.

Validation

• cargo test -p cortex-engine plan::tests -- --nocapture

Related bounty report: PlatformNetwork/bounty-challenge#11210

[greptile-apps]

Your free trial has ended. If you'd like to continue receiving code reviews, you can add a payment method here.