docs(using-git-worktrees): add subagent/executing-plans as callers

Update Integration section to show bidirectional relationship:
subagent-driven-development and executing-plans now list
using-git-worktrees as required, so this skill should list
them as callers.

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

skills/using-git-worktrees/SKILL.md

@@ -210,8 +210,9 @@ Ready to implement auth feature
 
 **Called by:**
 - **brainstorming** (Phase 4) - REQUIRED when design is approved and implementation follows
+- **subagent-driven-development** - REQUIRED before executing any tasks
+- **executing-plans** - REQUIRED before executing any tasks
 - Any skill needing isolated workspace
 
 **Pairs with:**
 - **finishing-a-development-branch** - REQUIRED for cleanup after work complete
-- **executing-plans** or **subagent-driven-development** - Work happens in this worktree