Drop registry-based group support from registry data

[rdimitrov]

Chore

Remove the ability to deploy server groups from registry data (thv group run). Registry-based groups were a ToolHive-specific concept embedded in registry JSON files that adds complexity without active use.

What

• groupRunCmdFunc returns clear error with migration guidance
• handleGroupLookup in retriever returns "no longer supported" error
• Delete ~400 lines of dead group deployment/validation code

Completion Checklist

• thv group run returns informative error pointing to thv group create + thv run --group
• Runtime groups (thv group create/list/rm) and groups API unaffected
• MCP optimizer continues to work (uses runtime groups, not registry groups)
• task build + task test + task lint pass