docs: Add playground demos skill

.agents/skills/playground-demos/SKILL.md

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+---
+name: playground-demos
+description: Choose and write live playground demos for the dataclient.io docs site and homepage (HooksPlayground examples, the homepage demo, their fixtures). Use when adding, redesigning or reviewing a docs demo, or coming up with a new demo idea for a feature.
+metadata:
+  internal: true
+---
+
+# Playground demos
+
+How playgrounds work is in the [Playground README](../../../website/src/components/Playground/README.md), mainly its Layouts and Design system sections.
+
+## Goal
+
+A demo soft-sells Data Client first and teaches users and agents how to use it second. People copy what they see and agents copy the code, so both teach.
+
+- **Use case**: one where the preview clearly shows the feature's advantage over the alternative.
+- **Mechanism**: easy to see how it works and why it's better.
+- **Look**: very beautiful, built from the playground design system, with no inline styles or CSS in the example code.
+- **Scope**: as simple as possible while still showing maximum value, utility and related features.
+- **New ideas**: inventing one needs at least high reasoning effort. At lower or unknown effort, hand the ideation to a high-effort subagent or ask the user.
+
+## Conventions
+
+- The demo makes its point without captions or narration text. If it needs them, pick another use case.
+- Loading states are skeletons (`<Loading />`, `skeleton`), not "Loading…" text.
+- A demo of a real public API calls its real URL, intercepted by a fixture in `website/src/fixtures/`, so copied code works.
+- The code follows the practices a careful app would, even where the demo could skip them, because readers copy it.
+- Use the `row` layout when the code is the lesson. Use `demo`, which hides the code, only when the point is purely visual, and ask the user first.
+- Anything extra a demo needs goes in the demo code with a design-system component, never a new Playground prop.
+- Check phone width and both themes.