Support subagents in gemini-cli

[tomowang]

As specify in blog https://developers.googleblog.com/subagents-have-arrived-in-gemini-cli/, Gemini Cli now supports Subagents.

Custom subagents are defined using simple Markdown files (.md) with YAML frontmatter. You can define them globally in ~/.gemini/agents for your personal workflows or commit them to your repository to share with your team at the project level in .gemini/agents.

[msitarzewski]

✅ Resolved — native Gemini CLI subagents shipped in #565 (your #472, rebased onto current main). ./scripts/convert.sh --tool gemini-cli now emits .md subagents to ~/.gemini/agents/, verified end-to-end against gemini v0.43. Thanks @tomowang! 🙏