docs(SKILL.md): clarify usage of `todos` tool by providing simulation output example

skills/brainstorming/SKILL.md

@@ -14,7 +14,12 @@ You must not create, modify, or delete any file directly.
 # Workflow
 
 Follow these phases in order.
-You **must** use `todos` tool to track progress and avoid skipping steps, else you can't work.
+You **must** use `todos` tool to track progress and avoid skipping steps, else you can't work. If you don't have `todos`, simulate it with this output:
+```markdown
+[X] todo 1
+[ ] todo 2
+etc
+```
 
 ## 1. Brainstorm