feat: add `rtk init hermes` for automatic plugin installation

Summary

Add Hermes to the list of agents supported by rtk init, enabling automatic plugin installation:

rtk init hermes
# or
rtk init -g --hermes

This would copy the Hermes plugin files (__init__.py + plugin.yaml) to ~/.hermes/plugins/rtk-rewrite/, similar to how rtk init installs hooks for Claude Code, Cursor, Copilot, etc.

Context

• The Hermes plugin was introduced in PR #1006 (hermes/ directory)
• Hermes is an AI coding agent by Nous Research with a Python plugin system
• Plugins live in ~/.hermes/plugins/<name>/ with a plugin.yaml manifest + __init__.py
• The plugin is also available via pip: pip install rtk-hermes (auto-registers via entry_points)

What rtk init hermes would do

1. Check if ~/.hermes/plugins/ exists (Hermes installation check)
2. Create ~/.hermes/plugins/rtk-rewrite/
3. Copy hermes/__init__.py and hermes/plugin.yaml into it
4. Print success message

Implementation notes

• Installation target: ~/.hermes/plugins/rtk-rewrite/
• The Rust code would live in src/hooks/init.rs alongside the other agent installers
• Detection: check for ~/.hermes/ directory or hermes binary in PATH
• Similar to the proposed rtk init --openclaw in #688

Related

• #1006 — Hermes plugin PR
• #688 — rtk init --openclaw request