feat: add rtk init -g --openclaw for automatic plugin installation

Summary

Add --openclaw flag to rtk init -g to automatically install the RTK plugin for OpenClaw, following the same pattern as --opencode.

Tasks

• Add --openclaw flag to rtk init CLI args
• Copy openclaw/index.ts + openclaw/openclaw.plugin.json to ~/.openclaw/extensions/rtk-rewrite/
• Add OpenClaw check to rtk init --check diagnostics
• Add uninstall support in rtk init --uninstall

Context

• OpenCode equivalent: rtk init -g --opencode (already implemented)
• OpenClaw plugin: PR #358
• Plugin location: ~/.openclaw/extensions/rtk-rewrite/

Reference

Pattern to follow: run_opencode_only_mode() in src/init.rs