feat(deploy): publish to GitHub Pages and Cloudflare Pages

Build the site from one source and deploy it to both hosts from GitHub
Actions, so neither depends on a host's default Jekyll toolchain.

- Gemfile/Gemfile.lock pin Jekyll 4.4.1 on Ruby 3.4; both workflows use
  them, which keeps the two sites on an identical toolchain
- _config.yml carries the GitHub Pages url and baseurl, fixing asset
  paths that previously resolved off-site; _config.cloudflare.yml layers
  over it for the pages.dev root
- wrangler.jsonc declares the Pages project name and output directory
- 404.html gives both hosts a page for unmatched routes
- exclude README.md from the output and stop shipping CSS sourcemaps

wrangler.jsonc

@@ -0,0 +1,9 @@
+{
+  // Cloudflare Pages project. The site is fully static — no Pages Functions —
+  // so this file only declares where Jekyll writes its output.
+  "name": "penny-pincher-provider",
+  "compatibility_date": "2026-08-28",
+
+  // Produced by `bundle exec jekyll build`; see docs/cloudflare-deployment.md
+  "pages_build_output_dir": "./_site"
+}