fix(ui-slash): make the reference lexicon reactive end to end

The decoration scan read a mutable lexicon() aggregation during render
with no subscription, so a catalog settling or a child spawning after
prewarm left drafted tokens undecorated until an unrelated re-render.
The controller now publishes the aggregation as a snapshot store fed by
a new optional SlashSource.subscribeLexicon hook (ui-skill notifies on
settle/invalidate, ui-subagent forwards the session-list feed), the
composer keyboard face exposes it as an observable, and InputBar
subscribes through uSES. Sources registered after scope birth now warm
and join live controllers via a service broadcast.

packages/client/ui-subagent/src/client/index.ts

@@ -39,6 +39,10 @@ export function apply(ctx: ClientContext): void {
       // The list snapshot is always warm — the full running-children roster.
       return childLabels(session, '')
     },
+    subscribeLexicon(_session, listener) {
+      // The roll derives from the list snapshot, so its change feed IS the list's.
+      return sessions.list.subscribe(listener)
+    },
     onPick({ candidate }) {
       // Decision 21: plain-text reference — the literal lands in the draft
       // and ships to the model verbatim (trailing space closes the token).