feat: add pull request template

Add structured PR template to standardize pull request format.
Template includes sections for description, decisions, benefits,
and a checklist for conventional commits and branch naming.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.gitea/PULL_REQUEST_TEMPLATE/default.md

@@ -0,0 +1,32 @@
+---
+title: "<type>: <description>"
+---
+
+## Description
+
+<!--
+First, describe the context for the PR.
+Then, explain why the PR exists.
+Finally, in concise, sentence-long bullets, explain each change.
+-->
+
+### Decisions
+
+<!--
+List any major architectural decisions here.
+If none exist, omit this section.
+-->
+
+## Benefits
+
+<!--
+List any major benefits here.
+How would this PR improve the codebase/product?
+-->
+
+## Checklist
+
+- [ ] Code follows conventional commit format.
+- [ ] Branch follows naming convention (`<type>/<description>`).
+- [ ] Tests pass (if applicable).
+- [ ] Documentation updated (if applicable).