feat: discover Kimi Code skills natively

A Kimi pane listed no skills. Reproduce Kimi Code's own resolution, checked
against its published skills documentation: a generic and a brand candidate
group at user and project scope, the generic group always first-existing, the
brand group governed by merge_all_available_skills, the brand copy winning a
name collision, project candidates resolved at the project root only, and
extra_skill_dirs added as additive user scope.

The config.toml parser reads the flat root-table subset those keys use and
leaves anything it does not understand alone, so a construct it cannot parse
never hides a command the user could have run.