Git doesn't track empty directories, so config/claude/skills was missing
from the Nix store source path, breaking home.file.".claude/skills"
evaluation. Add a .gitkeep to keep the directory present until real
skills are added.
Add a home-manager module that symlinks config/claude/commands and
config/claude/skills into ~/.claude, mirroring the opencode module.
Seed the commands directory with a /commit slash command.
