fix(plugins): composer commands run on current state, report failures, and reach the overflow sheet

Draft commands read the state captured at render, so a plugin's
`await setProvider(); await setModel()` acted on a stale provider. The draft
form and options are now plain stores that commands read live, and
`setProvider`/`setModel` reject an unavailable provider or a model the loaded
catalog does not list. Draft `options` include the plugin's pending edits.

Shared composer controls own the pending state and show a rejected command as
an error toast, like the built-in controls. Callback payloads are objects
(`{ value }`, `onPress(event)`) so they can grow without breaking plugins.

The overflow sheet's content portals to the app root and lost the composer's
agent, input, toast and query contexts, so plugin items drew nothing in it.
`useComposerSheetBridge` carries them across.

docs/plugins.md

@@ -663,6 +663,16 @@ density and overflow. Plugin items count in the density presence in `layout.ts`,
 condenses to make room for them. A custom component renders wherever its item is placed, and an
 item that renders nothing still counts for density; there is no registration for either.
 
+A command runs against the owner's current state, not the render that built the callback. The
+draft's form (`openAgentForm`) and options (`openDraftOptions`) are plain stores for that reason,
+and `draftComposerAgent` rejects a provider the host does not offer or a model its loaded catalog
+does not list. `ComposerControl` owns the pending state and the error toast for callbacks that
+return a promise.
+
+The overflow sheet's content portals to the app root, away from the composer. Pass
+`useComposerSheetBridge()` as the sheet's `contextBridge`, or `useComposer()` returns nothing and
+the sheet draws no plugin rows.
+
 ## Contribute timeline items
 
 Timeline transformers and renderers are client contributions. The daemon's canonical rows and