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
Add support for store directory paths provided as strings, in addition
to the existing path type check for directories.
Context: certain project will include a `SKILL.md` file, and instead of
managing them via some 3rd party manager or manually, why not include
them from the source itself! I implemented this in my setup, since I
wanted to include the `SKILL.md` file from the
[beads](https://github.com/steveyegge/beads) project.
