feat: separate merged from closed PRs, hide empty sections

[zeke]

This PR improves the activity report output:

• Separates "Merged Pull Requests" from "Closed Pull Requests" (unmerged)
• Hides section headings when there are no results

Testing locally

gh repo clone zeke/github-modules
cd github-modules/recent-github-activity
git checkout improve-pr-output
npm install
GITHUB_RECENT_ACTIVITY_PERSONAL_ACCESS_TOKEN=ghp_... npx . zeke