Enforce live plugin command activation

[TheGreatAxios]

Summary

• Disabled plugin skill commands are absent from discovery and cannot execute.
• Live enablement restores commands without restarting while built-in collision precedence remains intact.
• Slash command catalogs resolve current registry state instead of a startup snapshot.

Verification

• bun run check passes on the stacked branch
• Registry, command setup, and host tests pass

Fixes CL-7296

[linear-code]

CL-7296