chore(pages): deploy the demo from CI instead of by hand

[killerwolf]

Standardises this repo on the same Pages flow as QuickToss: deploy from main via GitHub Actions, rather than pushing a gh-pages branch from a laptop.

Why

npm run publish:demo made merging and deploying two separate events. Merging #40 did not change the live site — h4md1.fr/visual-image-tool/ kept serving the old demo page until publish:demo was run separately. A merged PR that silently leaves production stale is the failure mode worth removing.

It also published the working directory, not a commit. Whatever was on disk went live — dirty tree, wrong branch, unpulled changes — with nothing recording which commit the site corresponded to.

What it keeps

Both behaviours the old script had, because losing either would be silent:

• index-local.html stays unpublished. It loads ../dist/visual-image-tool.umd.js, which resolves above the site root once published and 404s. The old --src "**/!(index-local).*" filter excluded it; the workflow does the same with rsync --exclude.
• .nojekyll is written. Without it Pages runs Jekyll and drops any file whose name starts with an underscore.

Verified byte-identical before switching

I staged the workflow's output locally and diffed it against what gh-pages serves right now:

file list:  identical (12 files)
content:    all 11 published assets match SHA-256
            android-chrome-192x192.png, basic-usage.html, custom-config.html,
            demo-esm.html, demo-umd.html, favicon.ico, index.html,
            preact-importmap-demo.html, react-importmap-demo.html,
            social-preview.jpg, vue-importmap-demo.html

So the deploy is a no-op for visitors — same bytes, different delivery.

Also

• Drops the gh-pages devDependency and the publish:demo script.
• Rewrites the CONTRIBUTING section that documented the manual flow, including how to re-deploy without a code change (workflow_dispatch from the Actions tab).
• The CHANGELOG reference to publish:demo is left alone — it's a historical entry for a shipped release, and this changes nothing for anyone installing the package.

After merge

Pages source has to be flipped from gh-pages branch to GitHub Actions, then the gh-pages branch deleted. Doing that immediately after merging.

Full gate green: lint, format, types, 18 tests, build.

🤖 Generated with Claude Code