ci/redirects: Add Redirect check for PRs - #1880

Linear FE-499: Fix doc site issues

Stacked on #1858 (base branch check-links-pr-check); merge that one first.

Problem

• src/data/redirects.ts has ~900 entries that don't work: destination pages that no longer exist, #fragments in the source (browsers never send them, so the entry can never match), duplicates of an earlier source, chains to other redirects, /docs/ prefixes
• Nothing checks the file, so PRs keep adding broken redirects, and PRs that move a page or rename a heading silently break the redirects pointing at it

Solution

• dev/check-redirects.mjs validates every entry against the pages and headings in docs/
• "Check redirects" PR check runs the script on the PR head and its merge base, and reports only what the PR introduced; the ~900 pre-existing problems are ignored
• Runs in parallel with "Check links"
• The comment groups findings by problem, and each section says how to fix its entries, so authors learn how to write a correct redirect:
  • Source overshadows a docs page that exists
  • Source has a #fragment, so this redirect can never match
  • Source or destination starts with /docs
  • Source already has an earlier entry, so this one is never used
  • Destination is another redirect (shows the final destination)
  • Destination page does not exist
  • Destination heading does not exist
• Line numbers link to the entry in redirects.ts on the PR branch
• The comment is created / updated in place, and once the PR is fixed, the PR check passes and the comment is updated to say so
• A PR that never broke anything gets no comment
• pnpm check-redirects runs it locally

Verification

PR check comment in test PR: #1896 (comment)

Related

• ci/links: Add PR check for broken links #1858 adds the "Check links" PR check this is built on

Amp threads

• Redirect list cleanup
• Redirect check PR comment

Merge order for the PR-check stack

Trial-merged onto main in this order with no conflicts:

1. ci: Comment the Vercel build log on PRs whose build fails #1918 Vercel build log comment — independent; first so the other PRs' Vercel failures get a readable log
2. check-links: one suggestion per fix, synced with findings; one fact per line in reports #1916 check-links report format — adds dev/sync-review-comments.sh, which ci/redirects: Add Redirect check for PRs #1880 calls
3. ci/redirects: Add Redirect check for PRs #1880 redirect check — needs check-links: one suggestion per fix, synced with findings; one fact per line in reports #1916 merged first
4. spell check: update inline comments whose text changed #1919 spell check comment updates — independent
5. check-links: don't fail the generated-docs sync PR over absolute self-links it can't fix #1910 check-links, generated-docs sync PR — conflicts with check-links: one suggestion per fix, synced with findings; one fact per line in reports #1916 on dev/check-links.mjs; rebase after check-links: one suggestion per fix, synced with findings; one fact per line in reports #1916 merges

Squash-merge each, then rebase the next onto main.

#1920 (broken) and #1921 (fixed) are the example PRs that exercise every check; never merge, close them once the stack has landed.