Earlier quoted context omitted.
It shows up as a proper 301 when I load up the URL in Firefox. The question is, how?
One of the Jekyll plugins that GH Pages supports[0] is jekyll-redirect-from, which lets you put a `redirect_to` entry in a page's front matter. [0]: https://pages.github.com/versions/
~ % curl -i 'https://kotokaze.github.io/'
HTTP/2 301
server: GitHub.com
content-type: text/html
permissions-policy: interest-cohort=()
location: http://github.kotokaze.net/