fix(plugins): useComposers holds the composer it names, and a composer lives as long as its agent

PluginComposersSource.subscribe watched the stores directly and never the
composer model, so a component using useComposers did not keep the model's
catalog observer alive and reconnect repair skipped it. subscribe now takes the
selector and holds the model it names, following focus.

Dropping a composer when it went idle left a command a plugin had saved calling
a dead model after the tab reopened. A composer is now dropped when its agent
leaves the session and nothing watches it or has one mounted.

docs/plugins.md

@@ -679,9 +679,11 @@ implementations provide it:
   picker shows. The model loads the catalog itself and resolves with no models until it lands, as the picker does,
   so a read never differs from what the picker shows. While anything watches the model it holds a
   `QueryObserver` on the agent's catalog: that is what lets reconnect repair, which refetches only
-  active queries, refresh a catalog nobody has a picker open for, and retry one that failed. The
-  model is shared while a watcher or a mounted composer holds it and dropped when the last one
-  lets go. The mounted
+  active queries, refresh a catalog nobody has a picker open for, and retry one that failed.
+  `PluginComposersSource.subscribe(selector, listener)` is what holds it for `useComposers`, so
+  the subscription must go through the model's own `subscribe`. The model is shared for as long as
+  its agent is in the session, so a command a plugin saved keeps reaching the composer a later tab
+  mounts; it is dropped once the agent is gone and nothing watches it or has one mounted. The mounted
   `AgentControls` plugs its input in with `attachInput` so `insertText` lands at the cursor.
   Without a live input the model writes the stored prompt with `replaceDraftText`, which also asks
   a mounted input to show it. A workspace the user left stays mounted, display none, with no