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/tests/sessions-service.spec.ts

@@ -87,6 +87,23 @@ describe('list store projection', () => {
     expect(b.svc.list.getSnapshot().byId[sid('s1')]?.agentPreset).toBe('minimal')
   })
 
+  it('learns a preset switch from the host frame, not only from the tab that issued it', async () => {
+    const b = bench()
+    await feedList(b, [{ id: 's1', blank: true, agentPreset: 'standard' }])
+
+    // Every connected client gets this frame; only the switching tab gets the
+    // RPC echo. A client that ignored the payload would keep labelling the
+    // session with the composition it replaced.
+    b.svc.handleHostEnvelope({
+      rpcId: 'r1' as never,
+      payload: { type: 'host/session-preset-changed', sessionId: sid('s1'), agentPreset: 'minimal' } as never,
+    })
+    await Promise.resolve()
+
+    expect(b.svc.list.getSnapshot().byId[sid('s1')]?.agentPreset).toBe('minimal')
+    expect(b.svc.list.getSnapshot().byId[sid('s1')]?.blank).toBe(true)
+  })
+
   it('reflects live increments (host stream via manager) into the store', async () => {
     const b = bench()
     await feedList(b, [{ id: 's1' }])