Install skills and agents from packages

[gadenbuie]

We're adding support for skills and custom agents in

• feat: Subagent and Custom Agent Tools #149
• feat: implement Claude Skills #145

and there's an interesting opportunity to allow packages to publish and distribute these files, e.g. in inst/agents or inst/skills.

1. Could we easily auto-discover and attach an agent or skill from a package?
2. Could we offer a way to install agents/skills from a package into the project or user scope?
3. Should we auto-discover and use inst/agents and inst/skills when working inside an R package?