chore: add --body-file rule for PR descriptions

Author: jdsika

.github/copilot-instructions.md

@@ -71,6 +71,15 @@ When any feature branch is updated, rebuild `develop` from scratch.
 - **Target:** `linkml/linkml` `main`
 - **Source:** `ASCS-eV/linkml` `fix/...` or `feat/...`
 - **Never** open an upstream PR from `develop` or `main`
+- **Always** write the PR description to a markdown file first, then pass it
+  via `--body-file` to avoid shell escaping and formatting issues:
+
+```bash
+# Write description to .playground/pr-description.md first, then:
+gh pr create --repo linkml/linkml --base main --head ASCS-eV:fix/my-fix \
+  --title "fix(owlgen): description" \
+  --body-file .playground/pr-description.md
+```
 
 ### 5. Commit policy