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/runtime/src/client/sessions/manager.ts

@@ -780,6 +780,14 @@ export class SessionManager {
         }
         return
       }
+      case 'host/session-preset-changed': {
+        // Every connected client observes the switch here; only the tab that
+        // issued it also gets the RPC echo. The merge keeps the row's own
+        // updatedAt and lowers `blank` only, so re-applying the switching
+        // tab's own frame is a no-op.
+        this.noteAgentPreset(frame.sessionId, frame.agentPreset)
+        return
+      }
       case 'host/session-removed': {
         const summary = this.summaries.find(candidate => candidate.sessionId === frame.sessionId)
         const durableSubagent = summary?.origin === 'subagent' || this.addresses.has(frame.sessionId)