chore: add internal ui5wc-upgrade skill (#8511)

Author: Lukas742

skills/README.md

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+# Skills
+
+This directory contains agent skills for working with this repository.
+
+## Available Skills
+
+Currently, only **internal skills** for maintainers of this repository are offered.
+
+| Skill           | Description                                                                                                                                                                                         | Invocation              |
+| --------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------- |
+| `ui5wc-upgrade` | Guides a complete `@ui5/webcomponents` version upgrade — updates dependencies, regenerates wrappers, syncs theming parameters, detects new components and breaking changes, and verifies the build. | `/ui5wc-upgrade 2.25.0` |
+
+### Installing Internal Skills
+
+Since these skills are marked as internal, `npx skills` does not list them by default. To discover and install them, set the `INSTALL_INTERNAL_SKILLS` environment variable:
+
+```bash
+INSTALL_INTERNAL_SKILLS=1 npx skills add SAP/ui5-webcomponents-react --list
+```
+
+Alternatively, install from a local checkout:
+
+```bash
+INSTALL_INTERNAL_SKILLS=1 npx skills add ./skills --skill ui5wc-upgrade
+```
+
+## Requesting New Skills
+
+If you use an AI coding agent with UI5 Web Components for React and feel a skill would improve your workflow — whether for component usage, migration, styling, testing, or anything else — we'd love to hear about it.
+
+Additionally to the [`@ui5/webcomponents-react-mcp` MCP server](../packages/mcp-server/README.md), skills can provide guided multi-step workflows, bundled scripts, and domain-specific knowledge that go beyond what an MCP server offers.
+
+Open an issue describing your use case and we'll evaluate whether a public skill would be a good fit.