chore: add Copilot agent skills and instructions

.github/copilot-instructions.md

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+# Copilot Instructions for ui-theme
+
+## Project overview
+
+`@wintermuted/ui-theme` is a shared CSS/JS design-system package that publishes design tokens, base resets, and component styles. It is distributed via the GitHub Package Registry as an npm package.
+
+## Repository structure
+
+```
+styles/
+  tokens.css          # Design tokens (colours, spacing, typography, etc.)
+  base.css            # CSS reset and base element styles
+  components.css      # Barrel file that imports all component stylesheets
+  components/         # One CSS file per component (card, button, badge, …)
+index.css             # Public CSS entry point
+index.js              # Public JS entry point (re-exports styles as ES module)
+showcase/             # Static HTML/CSS documentation site for the component library
+scripts/              # Helper shell scripts (link-local, unlink-local)
+```
+
+## Coding conventions
+
+- All styling is plain CSS — no preprocessors (Sass, Less, PostCSS) are used.
+- Design tokens are defined as CSS custom properties in `styles/tokens.css` and must be used throughout component stylesheets instead of hard-coded values.
+- Each component lives in its own file under `styles/components/` and must also be registered in `styles/components.css` (barrel import) and in the `exports` map in `package.json`.
+- The showcase site uses `.component-index` + `.component-card` for card-style navigation tiles on documentation pages.
+- Versioning is fully automated: merging to `main` triggers `auto-release.yml`, which bumps the minor version, creates a tag, and publishes a GitHub Release. Do **not** manually edit the version in `package.json`.
+
+## Development workflow
+
+- Serve the showcase locally with `npm run showcase:serve` (starts a Python HTTP server on port 4174).
+- Link the package locally for integration testing with `npm run link:local` / `npm run unlink:local`.
+- There are no unit tests or linting scripts — validate changes by inspecting the showcase site.
+
+## Pull request guidelines
+
+- Keep component stylesheets focused and scoped; avoid cross-component style leakage.
+- When adding a new component, update `styles/components.css`, `package.json` exports, the showcase `components.html` page, and `showcase/index.html` navigation as needed.
+- Commit messages should follow conventional commits (`feat:`, `fix:`, `chore:`, `docs:`).

.github/workflows/copilot-setup-steps.yml

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+name: "Copilot Setup Steps"
+
+# Automatically run the setup steps when they are changed to allow for easy validation, and
+# allow manual testing through the repository's "Actions" tab
+on:
+  workflow_dispatch:
+  push:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+  pull_request:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+
+jobs:
+  # The job MUST be called `copilot-setup-steps` or it will not be picked up by Copilot.
+  copilot-setup-steps:
+    runs-on: ubuntu-latest
+
+    permissions:
+      contents: read
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: "22"
+
+      - name: Install dependencies
+        run: npm install --no-audit --no-fund