docs: update plan header to reference checkbox syntax

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

skills/writing-plans/SKILL.md

@@ -34,7 +34,7 @@ Assume they are a skilled developer, but know almost nothing about our toolset o
 ```markdown
 # [Feature Name] Implementation Plan
 
-> **For Claude:** REQUIRED: Use superpowers:subagent-driven-development (if subagents available) or superpowers:executing-plans to implement this plan.
+> **For Claude:** REQUIRED: Use superpowers:subagent-driven-development (if subagents available) or superpowers:executing-plans to implement this plan. Tasks and steps use checkbox (`- [ ]`) syntax for tracking.
 
 **Goal:** [One sentence describing what this builds]