Fix black-screen render: build with pandoc template

- Convert template.html to a pandoc template ($title$/$body$)
- Update CI deploy and docs to use --template template.html
- Fix header.html title to match canonical
- Ignore generated index.html (regenerated by CI)
- Update AGENTS.md build commands and file structure

.gitea/workflows/deploy.yml

@@ -15,7 +15,7 @@ jobs:
         run: apt-get update && apt-get install -y pandoc
 
       - name: Generate HTML
-        run: pandoc index.md -s -o index.html --css styles.css
+        run: pandoc index.md -s -o index.html --template template.html
 
       - name: Setup SSH key
         run: |