docs(subagent-driven-development): add using-git-worktrees as required skill

Adds using-git-worktrees as the first required workflow skill in the
Integration section. This makes explicit that an isolated workspace
should be set up before starting subagent-driven development.

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

skills/subagent-driven-development/SKILL.md

@@ -229,6 +229,7 @@ Done!
 ## Integration
 
 **Required workflow skills:**
+- **superpowers:using-git-worktrees** - REQUIRED: Set up isolated workspace before starting
 - **superpowers:writing-plans** - Creates the plan this skill executes
 - **superpowers:requesting-code-review** - Code review template for reviewer subagents
 - **superpowers:finishing-a-development-branch** - Complete development after all tasks