Add a module for Crush, Charm's coding agent for the terminal.
The module installs the package, writes the JSON configuration to
$XDG_CONFIG_HOME/crush/crush.json, and manages Agent Skills either from
an attribute set or from a directory. With `enableMcpIntegration` it
also takes the MCP servers from the shared `programs.mcp` module.
Co-authored-by: Rane <rane+git@junkyard.systems>
