Updates README.md and docs/reference.md with the new gt config command usage, including:
- All subcommands (agent list, get, set, remove, default-agent)
- Example of setting up a custom agent (claude-glm)
- Note about overriding built-in agents
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>