Feature request: Add Claude Code plugin support

## Feature Request

It would be great if this repository supported the [Claude Code](https://docs.anthropic.com/en/docs/claude-code) plugin system.

### Why

This repository already follows the [Agent Skills](https://agentskills.io/home) open standard with well-structured `SKILL.md` files. Adding a minimal `.claude-plugin/` configuration would allow Claude Code users to install this repository directly as a plugin and access all the Android skills within their development workflow.

### What's needed

Just two small JSON files in a `.claude-plugin/` directory:

- **`plugin.json`** — Plugin metadata (name, version, author, license, skill discovery path)
- **`marketplace.json`** — Marketplace entry point for plugin discovery and installation

Since the skills are organized in category-based subdirectories (e.g., `build/agp/`, `jetpack-compose/migration/`), the `plugin.json` would use `"skills": "./"` to scan from the repository root — no restructuring required.

### Reference implementation

I've prepared a working implementation in my fork:
https://github.com/dandelion0216/skills/tree/add-claude-code-plugin-config/.claude-plugin

This is purely additive — no existing files are modified.

Start by reviewing the referenced fork's `.claude-plugin` directory and the repository's existing `SKILL.md` layout. Add `.claude-plugin/plugin.json` and `.claude-plugin/marketplace.json` with the requested metadata, root skill discovery, and marketplace entry so Claude Code users can install the repository without changing existing files.

Written by the indexing model from the issue text.