feat(plugin): enable/disable plugins from the Plugin Manager

The manager was a read-only inventory. Each plugin now has an
Enable/Disable toggle: a disabled plugin keeps its manifest listed but
drops its ribbon tab and command dispatch. The disabled-id set persists
in settings.txt (disabled_plugins=) and the ribbon rebuilds on toggle.

Part of the #100 extensibility epic.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/plugin/mod.rs

@@ -8,6 +8,6 @@ pub mod host;
 pub mod manifest;
 pub mod registry;
 
-pub use registry::all_ribbon_modules;
+pub use registry::{all_ribbon_modules, ribbon_modules_enabled};
 pub(crate) use host::BuiltinPlugin;
 pub(crate) use registry::{installed_manifests, try_dispatch};