ScriptKit: on-demand motd command with debounced startup delivery

- !<plugin> motd alias: show a random tip for that plugin
- !scriptkit motd [plugin]: global pool or plugin-specific
- In-memory no-repeat tracking (resets when all shown)
- Debounced startup: single tip 10s after last registration (replaces per-plugin)
- Derived button styling from frame borderLeft accent color
- motdButtonStyle opt for explicit override