Update pr-drafting.md

Author: coisa

.agents/skills/fast-forward-github-pull-request/references/pr-drafting.md

@@ -27,6 +27,20 @@ If no PR template exists, use the fallback structure below.
 
 ## Fallback PR Body
 
+```markdown
+## Summary
+[One short paragraph describing the implemented behavior]
+
+## Changes
+- [Concrete change]
+- [Concrete change]
+
+## Testing
+- [Command and result]
+- [Command and result]
+
+Closes #123
+```
 
 ## Title Guidance