docs(sdd, executing-plans): update worktree Integration descriptions (PRI-823)

Clarify that using-git-worktrees ensures a workspace exists rather than
always creating one.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

skills/executing-plans/SKILL.md

@@ -65,6 +65,6 @@ After all tasks complete and verified:
 ## Integration
 
 **Required workflow skills:**
-- **superpowers:using-git-worktrees** - REQUIRED: Set up isolated workspace before starting
+- **superpowers:using-git-worktrees** - REQUIRED: Ensures isolated workspace (creates one or verifies existing)
 - **superpowers:writing-plans** - Creates the plan this skill executes
 - **superpowers:finishing-a-development-branch** - Complete development after all tasks

skills/subagent-driven-development/SKILL.md

@@ -265,7 +265,7 @@ Done!
 ## Integration
 
 **Required workflow skills:**
-- **superpowers:using-git-worktrees** - REQUIRED: Set up isolated workspace before starting
+- **superpowers:using-git-worktrees** - REQUIRED: Ensures isolated workspace (creates one or verifies existing)
 - **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