feat: Restructure as Claude Code plugin for marketplace installation

[richzw]

The previous format required claude skill add --url ... which is being superseded by the plugin system.

The plugin format supports marketplace discovery, versioning, and the standard /plugin install workflow.

---

Change Summary

• Restructure repository from standalone skill format to Claude Code plugin format, enabling installation via the plugin marketplace (/plugin install)
• Add .claude-plugin/plugin.json manifest with plugin metadata (name, version, author, keywords)
• Move SKILL.md and references/ into skills/milvus/ following the plugin directory convention