fix(plugins): composer option resets and model checks match the documented commands

A draft kept `null` as an option value, so `setOptions({ key: null })` sent the null to the
provider and creation failed. A null edit now removes the key from every draft layer, as the patch
does on a running agent. A running agent's `setModel` accepted ids its catalog does not list; it
now rejects them the way a draft does.

The composer controls imported the toast host through the toast context, which pulled the host's
animation module into every module that loads plugins. They use the API-only context instead.
The migration guide's registration table gets the `addComposerItem` row its test requires.

packages/app/src/plugins/composer-items/index.tsx

@@ -8,7 +8,7 @@ import {
 } from "@/composer/agent-controls/composer-control";
 import { useComposerAgent, useComposerInput } from "@/composer/session";
 import { useIsCompactFormFactor } from "@/constants/layout";
-import { useToast } from "@/contexts/toast-context";
+import { useToast } from "@/contexts/toast-api-context";
 import { useHosts } from "@/runtime/host-runtime";
 import type { Theme } from "@/styles/theme";
 import { createPluginClientStateSource } from "../client-state/source";