feat: Add `~/.agents/skills` as a global skill directory

[0be30301]

Currently, ~/.agents/skills is only supported as a workspace-relative path (.agents/skills under the project root). This means a user cannot place a skill in ~/.agents/skills/ and have it discovered across all projects. The only global directory is ~/.cptr/skills.

Other agnostic agents use ~/.agents/skills as their global skill directory, this would reduce yet another skills folder to manage.