Add headlamp-plugin-developer agent skill

Extracted from headlamp-sealed-secrets-plugin to serve as the
standalone home for developing Headlamp agent skills.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

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+# Headlamp Agent Skills
+
+A collection of Claude Code agent skills for developing [Headlamp](https://headlamp.dev/) Kubernetes dashboard plugins.
+
+## Available Skills
+
+### headlamp-plugin-developer
+
+A senior Headlamp plugin engineer agent that covers:
+
+- Plugin registration APIs (`registerRoute`, `registerSidebarEntry`, `registerDetailsViewSection`, etc.)
+- CommonComponents usage (`SectionBox`, `SimpleTable`, `NameValueTable`, `StatusLabel`, etc.)
+- K8s module and KubeObject patterns
+- CRD class definitions
+- ApiProxy usage and service proxy URLs
+- Theming and dark mode with CSS variables
+- Testing mocks for Headlamp APIs
+- Code quality conventions (TypeScript strict mode, functional components, accessibility)
+
+## Installation
+
+Copy the desired agent skill(s) from `.claude/agents/` into your project's `.claude/agents/` directory:
+
+```bash
+cp .claude/agents/headlamp-plugin-developer.md /path/to/your-headlamp-plugin/.claude/agents/
+```
+
+## Usage
+
+Once installed in your Headlamp plugin project, Claude Code will automatically use the agent when building, extending, debugging, or reviewing Headlamp plugins.
+
+## Contributing
+
+Contributions are welcome! To improve an existing skill or add a new one:
+
+1. Edit or create a markdown file in `.claude/agents/`
+2. Follow the existing format (YAML frontmatter + detailed reference sections)
+3. Test the skill against a real Headlamp plugin project
+4. Submit a pull request