RFC: Skills Lifecycle Management in ToolHive CLI

This RFC proposes adding skill lifecycle management capabilities to
the ToolHive CLI (thv), enabling users to install, remove, list, and
package Agent Skills as OCI artifacts.

Key decisions:
- Dedicated `thv skill` command namespace
- Extend pkg/client/ for skills (convergence with MCP client system)
- Hybrid state storage (per-skill metadata + index file)
- Integration with groups system
- Require explicit --client selection

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: JAORMX