Catalog: apps, featured order, validator with tests, submission guide

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/PULL_REQUEST_TEMPLATE.md

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+## App
+
+What the app does, and how far along it is.
+
+- [ ] The folder and `id` are lowercase kebab-case and match.
+- [ ] `author`, `description` and `website` or `source` are set.
+- [ ] Every preview is a PNG in the app folder, the whole window in each.
+- [ ] Any README describes the product, links only to the website or source, and stays under 10 KB.
+- [ ] `featured`, `publishedAt` and stars are left to maintainers and CI.
+- [ ] `pnpm test` and `pnpm validate` pass.