fix: make dev up, dev server, and dev docs work

[kdaviduik]

Why

dev up was broken (the node task used a pnpm: key that current dev rejects), dev server failed to bind port 8080 (devns conflict), and dev docs had no Ruby/Jekyll toolchain provisioned. This PR fixes all three so the standard dev workflow works out of the box — no manual setup steps.

What

dev up — fixed node task + provisioned Jekyll

• dev.yml node task: pnpm: 10.16.1 → package_manager: pnpm@10.16.1 + packages: [.] (the old pnpm: key was rejected by dev's validator)
• Added ruby + bundler to up: — provisions Ruby 3.3.6 and Jekyll 4.4.1 so dev docs works without manually installing Jekyll
• New files: .ruby-version (3.3.6), Gemfile (gem 'jekyll', '~> 4.4'), Gemfile.lock (generated by dev up)

dev server — renamed command, fixed port binding, removed manual setup

• Renamed start → server in dev.yml (matches the convention across Shopify repos)

• server now runs pnpm run build && pnpm run start — a restart picks up source changes cleanly

• Renamed open: example → open: app — Ctrl-T during dev server is hardcoded to look for a link named app

• Fixed EADDRINUSE on port 8080: changed serve script from http-server to http-server -a 127.0.0.1 -p 8080

  devns (dev's networking daemon) holds 0.0.0.0:8080 in a CLOSED socket state to proxy *.shop.dev traffic. http-server defaults to binding 0.0.0.0:8080, which conflicts. Binding to 127.0.0.1 avoids the conflict. -p 8080 is also needed — without it, http-server uses portfinder which checks 0.0.0.0 (still blocked by devns) and auto-increments to 8081+.

  Evidence: http-server README documents that -a defaults to 0.0.0.0 and -p defaults to 8080. You can reproduce the conflict by running node -e "require('http').createServer().listen(8080, '0.0.0.0')" — it throws EADDRINUSE, while listen(8080, '127.0.0.1') succeeds.

  Why not -p 0 (auto-find open port)? The http-server docs suggest -p 0 to let portfinder find an open port starting at 8080. However, portfinder checks availability on 0.0.0.0 (where devns holds the CLOSED socket), not on the -a 127.0.0.1 address — so it sees 8080 as taken and skips to 8081, which breaks dev open app (hardcoded to http://localhost:8080/). Using an explicit -p 8080 bypasses portfinder entirely and binds directly to 127.0.0.1:8080.

• Renamed index.example.html → index.html and hardcoded the hydrogen-preview demo store credentials (same public credentials already used in test-manual/index.html). Removed /index.html from .gitignore so it's committed. This eliminates the manual cp + credential-filling step — dev server just works.

  Why this is safe: index.html was originally gitignored in 2016 because it contained a real Storefront API key. The example/template pattern was the 2016 solution to "don't commit API keys." This concern no longer applies: storefront access tokens are public-by-design (they're embedded in browser-side JS on merchant websites — that's literally what buy-button-js does), they only grant read access to products/collections, and the specific credentials used here are already committed in test-manual/index.html in this same repo.

dev docs — fixed docs command + stale docs

• docs script: jekyll serve → bundle exec jekyll serve (uses the Jekyll provisioned by dev up)
• docs/readme.md: was copy-pasted from the js-buy-sdk repo (wrong title, wrong URL, referenced non-existent doc-build/doc-serve scripts) — rewritten to accurately describe this repo's docs setup
• readme.md: Documentation section no longer says to manually gem install jekyll (dev up handles it); corrected "gh-pages from main" to "Pages builds /docs from main" (no gh-pages branch); removed manual cp index.example.html index.html step

Ignore files

• .gitignore: removed /index.html (now committed); added .jekyll-cache (Jekyll 4 cache dir)
• .npmignore: excluded /Gemfile, /Gemfile.lock, /.ruby-version so Ruby tooling doesn't ship to npm consumers (index.html was already excluded)

How to verify

dev up          # should converge cleanly (Ruby + Jekyll installed)
dev server      # should build, then serve at http://localhost:8080/ (demo page with hydrogen-preview store)
dev docs        # should serve docs at http://localhost:4000/buy-button-js/