Remove session scope language from SUBAGENT-STOP

Drop 'top-level sessions only' which may cause the model to treat
using-superpowers as a first-turn-only skill, breaking skill chaining
on follow-up turns.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

skills/using-superpowers/SKILL.md

@@ -4,7 +4,7 @@ description: Use when starting any conversation - establishes how to find and us
 ---
 
 <SUBAGENT-STOP>
-If you were dispatched as a subagent to execute a specific task, skip this skill. It is for top-level sessions only.
+If you were dispatched as a subagent to execute a specific task, skip this skill.
 </SUBAGENT-STOP>
 
 <EXTREMELY-IMPORTANT>