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

header.html

@@ -1,3 +1,3 @@
 <div class="header">
-  <h1>I want to be a dad</h1>
+  <h1>I want to be a dad!</h1>
 </div>