[mikkel]

What

This redirects every page of gptdiff.255labs.xyz to the matching page under https://patchling.app/py/. It's off by default, behind a repository variable.

How gptdiff is served: GitHub Pages. The Deploy Documentation workflow runs actions/deploy-pages and writes public/CNAME. GitHub Pages has no _redirects or server-side redirects. So the deploy job now rewrites the built site into redirect stubs. No Cloudflare or DNS rule is needed.

• New scripts/gptdiff_redirect_stubs.py public https://patchling.app/py/. It turns every built page into a stub with:

  • <link rel="canonical">;
  • <meta name="robots" content="noindex">;
  • a 0-second meta refresh;
  • location.replace(target + location.search + location.hash), so #anchors survive.

  404.html maps any other request in JS:

  • known pages, with or without a trailing slash or index.html, go to the same path under /py/;
  • unknown paths go to /py/.

  Everything else is removed except CNAME: assets, search index, sitemaps.

• documentation.yml: a new step after "Ensure CNAME is set", only if vars.GPTDIFF_REDIRECT_TO_PATCHLING_APP == 'true'.

• The mkdocs source is untouched. patchling.app's build (site: serve patchling-py docs at /py/ and JS docs at /js/ on patchling.app patchling#13) clones this repo and still builds the real docs.

Merge order (don't redirect before /py is live)

1. site: serve patchling-py docs at /py/ and JS docs at /js/ on patchling.app patchling#13 first: apply its Pages build settings (build command bash scripts/build-site.sh, output dist, PYTHON_VERSION=3.12), then merge. Check that https://patchling.app/py/ and https://patchling.app/py/cli/ load.
2. This PR can merge any time. Without the variable, the deploy is unchanged and gptdiff keeps serving the real docs.
3. Switch on: GitHub → 255BITS/patchling-py → Settings → Secrets and variables → Actions → Variables tab → New repository variable. Name GPTDIFF_REDIRECT_TO_PATCHLING_APP, value true. Then Actions → Deploy Documentation → Run workflow on main, or wait for the next push.
4. Rollback: delete the variable (or set it to anything else) and re-run Deploy Documentation.

#28 (deploy hook) is independent of this PR. Merge order doesn't matter between them.

Tests (local)

• actionlint 1.7.12 with shellcheck and yamllint on documentation.yml: clean.
• Offline pytest: 210 passed, 2 skipped, including 3 new tests in tests/test_gptdiff_redirect_stubs.py:
  • path mapping, canonical and meta refresh;
  • the 404.html known list and fallback;
  • only the stubs and CNAME remain.
• End to end: a real mkdocs build --strict of main (9555eb8) was converted to stubs pointing at a local build of patchling.app (dist/ from Relevance filter: Ember review follow-ups #13). It was served by a server that behaves like GitHub Pages (404.html for unknown paths, /dir → /dir/), and Chromium followed the redirects. All 10 cases landed on the right page:
  • / → /py/
  • /cli, /cli/ and /cli/index.html → /py/cli/
  • /cli/#environment-variables → /py/cli/#environment-variables
  • /api/?q=1#smartapply → /py/api/#smartapply (mkdocs-material's search consumes ?q= on arrival)
  • /examples/basic → /py/examples/basic/
  • /quickstart/index.html#install → /py/quickstart/#install
  • /no/such/page/ → /py/

Search engines see real 404s with a JS and meta redirect, and 200 stubs with rel=canonical plus noindex. A true HTTP 301 would need a Cloudflare redirect rule on 255labs.xyz, if that zone is proxied; this PR changes nothing there.