fix(desktop): guard setPrimaryGatewayConnectionId against non-primary active scopes

Hardening follow-up to #95396 (#95628): ignore primary connection-id writes
while the active key is a composite secondary scope, so future
presentation-layer writes cannot relabel the primary socket and poison
new-session routing. Regression test is sabotage-proven (fails with the
guard reverted).

apps/desktop/src/store/gateway-connection-scope.test.ts

@@ -45,6 +45,7 @@ const {
   closeSecondaryGateways,
   configureGatewayRegistry,
   ensureGatewayForAgent,
+  ensureGatewayForProfile,
   openGatewayForAgent,
   pruneSecondaryGateways,
   setPrimaryGateway,
@@ -106,6 +107,25 @@ describe('primary gateway registry scope', () => {
     expect(activeGatewayConnectionId()).toBeNull()
     expect(setApiRequestConnection).toHaveBeenLastCalledWith(null)
   })
+
+  it('ignores primary connection-id writes while a secondary registry scope is active (#95628 hardening)', async () => {
+    setPrimaryGateway({ connectionState: 'open' } as never, 'default')
+    setPrimaryGatewayConnectionId('primary-vps')
+
+    // Foreground Gateway B's composite scope (connectionId 'homelab').
+    await expect(ensureGatewayForAgent('homelab', 'default')).resolves.toBe(true)
+
+    // Presentation-layer write while the secondary is foregrounded: the id
+    // describes the secondary, not the primary. It must be dropped — accepting
+    // it relabels the primary socket and poisons ambient routing.
+    setPrimaryGatewayConnectionId('homelab')
+
+    // Back on the primary route, its registry identity is intact.
+    await ensureGatewayForProfile('default')
+
+    expect(activeGatewayConnectionId()).toBe('primary-vps')
+    expect(setApiRequestConnection).toHaveBeenLastCalledWith('primary-vps')
+  })
 })
 
 describe('pruneSecondaryGateways with registry-scoped entries', () => {

apps/desktop/src/store/gateway.ts

@@ -261,6 +261,17 @@ export function setPrimaryGateway(gateway: HermesGateway | null, profile = 'defa
 }
 
 export function setPrimaryGatewayConnectionId(connectionId: null | string | undefined): void {
+  // Hardening for #95628: while the active route is a secondary scope, the
+  // window is looking at a NON-primary socket — any connection id flowing
+  // through presentation-layer code at that moment describes the secondary,
+  // not the primary. Accepting it would relabel the primary socket, so every
+  // ambient API/WebSocket helper (and new-session routing) silently lands on
+  // the wrong backend. The primary's own identity is (re)published by its
+  // boot/reconnect path, which runs with the primary route active.
+  if (!isActivePrimary()) {
+    return
+  }
+
   g.primaryConnectionId = (connectionId ?? '').trim() || null
 
   if (g.activeKey === g.primaryProfile) {