The skills option was creating files under ~/.config/opencode/skill/
(singular) but OpenCode documentation only mentions
~/.config/opencode/skills/ (plural). Both work, but using an
undocumented directory can be confusing.
Fixes: https://github.com/nix-community/home-manager/issues/8907
Adds support for OpenCode Agent Skills by managing skill definitions
under "/opencode/skill/<name>/SKILL.md" via `programs.opencode.skills`.
Documentation: https://opencode.ai/docs/skills/