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>

Author: JAORMX

rfcs/THV-0030-skills-lifecycle-management.md

@@ -335,7 +335,7 @@ Skills integrate with ToolHive's existing groups system:
 **Behavior:**
 - `thv skill install --group development` adds skill to group
 - `thv skill uninstall` removes skill from any groups
-- `thv group list` shows skills alongside workloads
+- `thv skill list --group <name>` shows skills installed in a group
 
 ### OCI Artifact Format