fix(web): fold the preset frame into the session row for every client

The frame carried `agentPreset` for surfaces that label the session, but
nothing consumed it: `noteAgentPreset` ran only in the switching tab's RPC
callback, so a second connected client refetched its catalogs while its
session row — the header label's source, and the hero chip's no-op input —
kept the composition the session had replaced.

`SessionManager.handleHostEnvelope` now folds the frame like the other
session frames. Re-applying the switching tab's own frame is a no-op: the
merge lowers `blank` only and keeps the row's `updatedAt`.

packages/client/ui-command/src/client/service.ts

@@ -124,7 +124,7 @@ export class CommandService extends Service implements CommandServiceContract {
       warm: (session) => { this.directory.warm(session.sessionId) },
     }), 'command: slash source')
     ctx.on('commands/changed', () => { this.directory.invalidateAll() })
-    // A preset switch changes which commands ONE session's agent resolves and
+    // A preset switch changes which commands one session's agent resolves and
     // registers nothing globally, so the registry-wide signal above never
     // fires for it: repull that key alone, soft, so the old snapshot serves
     // the menu until the new one lands.