Add AGENTS.md with project guidelines for opencode

AGENTS.md

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+# rusty-petroleum Project Guidelines
+
+## Git Commit Style
+- Make small, focused commits while working
+- Commit changes incrementally to make history easier to follow
+- Each commit should represent a single logical change
+- Write clear, concise commit messages describing what changed
+
+## Issue Management
+- Use Gitea REST API to close issues when fixing them:
+  ```bash
+  curl -X PATCH https://gitea.rowanbrook.net/api/v1/repos/jakob/rusty-petroleum/issues/{issue_number} \
+    -H "Content-Type: application/json" \
+    -H "Authorization: Bearer YOUR_TOKEN" \
+    -d '{"state": "closed"}'
+  ```
+- Or suggest closing issues on Gitea web UI when automatic closing via API is not configured