RFC: Skills Lifecycle Management in ToolHive CLI (#30)

* 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>

* Rename RFC to THV-0030 to match PR number

* Update RFC title to RFC-0030

* Use thv skill list --group for group filtering

Address PR feedback: instead of modifying thv group list behavior,
add --group flag to existing thv skill list command for filtering
skills by group.

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

* Address PR feedback on RFC-0030

- Add -u/--upgrade flag to install command for skill upgrades
- Add thv skill validate command for pre-build validation
- Expand client support table with all Skillet clients (Claude, Codex, Copilot, OpenCode)
- Clarify OCI artifact format (not runnable containers, uses OCI for distribution)
- Explain deterministic packaging rationale for supply chain security
- Expand Alternative 5 rationale for requiring explicit --client selection
- Note that plugins cache is out of scope (managed by Claude's plugin system)

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

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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: JAORMX