Improve PR template with project section and clarified checklist

Add dedicated Project section at top for contributors to specify
project name and URL upfront. Clarify checklist item from "One link
per Pull Request" to "One project per PR" for better consistency
with PR title format guideline.

🤖 Generated with Claude Code (https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>

Author: vinta

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,10 @@
+## Project
+
+[Project Name](url)
+
 ## Checklist
 
-- [ ] One link per Pull Request
+- [ ] One project per PR
 - [ ] PR title format: `Add project-name`
 - [ ] Entry format: `* [project-name](url) - Description ending with period.`
 - [ ] Description is concise and short