feat(hermes): memory enable switch + clearer migration warning copy

Replaces the greyed-out "Memory is disabled" banner with a real Switch
at the top of each memory tab. Users can now toggle Hermes' memory/user
blobs without leaving CC Switch; the underlying write goes through the
merge-aware `set_memory_enabled`, so budgets and external-provider
settings survive toggle operations. The new `useToggleHermesMemoryEnabled`
mutation invalidates the limits query so the Switch state and the
amber disabled-hint update in lockstep.

Reworks the `schema_migrated_v12` health banner copy to match the
simplified "CC Switch only manages custom_providers" posture — it now
tells users to reconcile migrated dict entries via Hermes Web UI,
instead of the earlier (and now inaccurate) "CC Switch reads both".

src/hooks/useHermes.ts

@@ -125,6 +125,34 @@ export function useSaveHermesMemory() {
   });
 }
 
+/**
+ * Toggle one memory blob's on/off flag in Hermes' `config.yaml`. Invalidates
+ * the limits query so the switch UI and disabled banner update immediately.
+ */
+export function useToggleHermesMemoryEnabled() {
+  const queryClient = useQueryClient();
+  const { t } = useTranslation();
+  return useMutation({
+    mutationFn: ({
+      kind,
+      enabled,
+    }: {
+      kind: HermesMemoryKind;
+      enabled: boolean;
+    }) => hermesApi.setMemoryEnabled(kind, enabled),
+    onSuccess: async () => {
+      await queryClient.invalidateQueries({
+        queryKey: hermesKeys.memoryLimits,
+      });
+    },
+    onError: (error) => {
+      toast.error(t("hermes.memory.toggleFailed"), {
+        description: extractErrorMessage(error) || undefined,
+      });
+    },
+  });
+}
+
 /**
  * Returns a handler that probes the local Hermes Web UI, opens it in the
  * system browser, and surfaces a localized toast on failure. Callers only