feat(web): add Skill Playground — browser UI to run any skill with your own key

A zero-backend static web app to run any of the 172 skills directly in the
browser using the user's own Claude API key (stored only in localStorage,
sent straight to api.anthropic.com).

- build-skills.mjs: generates skills.json from skills/*/SKILL.md, parsing
  frontmatter, the Required Inputs section (-> form fields), and a one-line
  summary for each skill tile.
- Tile gallery with bundle tag, title, and one-line description; search +
  bundle filter; click a tile to open an auto-generated input form.
- Streams output via the Anthropic Messages API (direct browser access),
  with copy/download, model picker, and Show/Hide key toggle.
- Product Notes logo in the header.
- README: add Skill Playground section + screenshot, a table of contents,
  and collapse the long changelog and full skills list into <details> blocks.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/README.md

@@ -0,0 +1,36 @@
+# Skill Playground
+
+A zero-backend web app to run any skill in this repo with **your own Claude API key**.
+Pick a skill → it becomes a form → fill it in → Claude executes the skill's instructions
+and streams the result. Your key is stored only in your browser (`localStorage`) and sent
+directly to `api.anthropic.com`. Nothing touches a server we own.
+
+## Run locally
+
+```bash
+node web/build-skills.mjs        # regenerate skills.json from skills/
+cd web && python3 -m http.server 8000
+# open http://localhost:8000
+```
+
+> It must be served over HTTP (not opened as a `file://` URL) so `fetch('skills.json')` works.
+
+Paste a key from [console.anthropic.com](https://console.anthropic.com/settings/keys) and run.
+
+## How it works
+
+- `build-skills.mjs` scans `../skills/*/SKILL.md`, parses the frontmatter and the
+  **Required Inputs** section, and writes `skills.json` (the UI's data source).
+- `app.js` sends the skill's instruction body as the `system` prompt and the filled-in
+  fields as the user message, using the Anthropic Messages API with
+  `anthropic-dangerous-direct-browser-access: true` for direct browser calls.
+
+## Keep it in sync
+
+Re-run `node web/build-skills.mjs` whenever skills are added or edited, and commit the
+updated `skills.json`. (Or wire it into CI / a pre-commit hook.)
+
+## Deploy
+
+It's fully static — host the `web/` folder on GitHub Pages, Netlify, Vercel, or any
+static host. No environment variables, no server.