create-pr skill: prohibit Test plan section

Documentation PRs don't need test plans, but the system-level PR
template includes one. Add an explicit prohibition so the skill
instruction overrides the default.

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

Author: dvdksn

.agents/skills/create-pr/SKILL.md

@@ -69,7 +69,7 @@ EOF
 ```
 
 Keep the body short. Reviewers need to know what changed and why — nothing
-else.
+else. Do **not** add a "Test plan" section — documentation PRs don't need one.
 
 Use an accurate disclosure footer that names the active coding agent, for
 example `Generated by Codex` or `Generated by Claude Code`.