Proposal: adopt XDG Base Directory paths for global skills

Issue #15 proposes .agents/skills as a common skills directory, and many agent clients have adopted it. Most popular clients (except Claude Code) also accept ~/.agents/skills for global skills.

This proposal concerns that global location. On Unix-like systems, clients should support an XDG Base Directory path instead of requiring another top-level hidden directory in $HOME. The project-local .agents/skills directory would remain unchanged.

XDG support has already been discussed several times in #15. Three approaches have been suggested:

1. $XDG_CONFIG_HOME/agents/skills
2. $XDG_DATA_HOME/agents/skills
3. Both locations, with a documented search order

I prefer $XDG_DATA_HOME/agents/skills, falling back to $HOME/.local/share/agents/skills when $XDG_DATA_HOME is unset.

The XDG Base Directory Specification defines $XDG_DATA_HOME for user-specific data and $XDG_CONFIG_HOME for configuration. Skills are content rather than settings and may include scripts, documentation, and large reference files, so the data directory seems like the better fit.

For backward compatibility, clients could still search ~/.agents/skills after the XDG location.

Please leave a if you agree with this proposal. Reactions help demonstrate demand for a standardized XDG-compatible global skills location.