A test proves every top-level page and a repository, issue, pull and person page carry og and twitter tags #2446

Part 1 of #2349.

Done looks like:

• [ ] One new test file fetches each top-level page, plus one repository, issue, pull and person page. On each it checks for og:title, an og:description that is not empty, an og:image that answers 200 image/png, and twitter:card=summary_large_image (src/web/layout.ts emits them).
• [ ] Any page the test finds missing a tag is fixed in the same pull and named in it.
• [ ] Run the new file plus test/og-route.test.ts and quote the result. Pull #2379 went red on a test it never touched ("working now in the terminal"). If that happens again, name it in the pull and leave it alone.