base-layout.txt
My site is <one sentence>, built with <stack> and hosted on <host>. Every
page currently repeats its own header and footer.

Create one shared layout, done this stack's usual way:
- A proper document head: title, description, social tags, canonical URL.
- Header with navigation, marking the current page.
- A single main area each page fills in.
- Footer.
- Accessible from the start: a skip link, landmarks, one h1 per page.

Then convert one existing page to use it, and show me the pattern for
the rest.

Do this before the site has twenty pages. Converting two is ten minutes. Converting twenty is an afternoon of spotting which ones drifted.