fix(app): replace greedy host runtime subscriptions with targeted hooks

useHostRuntimeSession returned the full HostRuntimeSnapshot, causing
every consumer to rerender on any internal state change (probe cycles,
client generation bumps). Replace with targeted hooks that return
primitives/stable references so useSyncExternalStore skips rerenders
when the subscribed value hasn't actually changed.

New hooks: useHostRuntimeClient, useHostRuntimeConnectionStatus,
useHostRuntimeLastError, useHostRuntimeAgentDirectoryStatus,
useHostRuntimeIsDirectoryLoading. Existing useHostRuntimeIsConnected
already followed this pattern.

useHostRuntimeSnapshot kept only for settings-screen where probe
data is legitimately needed.

packages/app/src/hooks/use-agent-form-state.ts

@@ -10,7 +10,7 @@ import type {
   AgentProvider,
 } from "@server/server/agent/agent-sdk-types";
 import { useHosts } from "@/runtime/host-runtime";
-import { useHostRuntimeSession } from "@/runtime/host-runtime";
+import { useHostRuntimeClient, useHostRuntimeIsConnected } from "@/runtime/host-runtime";
 import { useFormPreferences, type FormPreferences } from "./use-form-preferences";
 
 // Explicit overrides from URL params or "New Agent" button
@@ -367,7 +367,8 @@ export function useAgentFormState(
   }, [isVisible]);
 
   // Session state for provider model listing
-  const { client, isConnected } = useHostRuntimeSession(formState.serverId ?? "");
+  const client = useHostRuntimeClient(formState.serverId ?? "");
+  const isConnected = useHostRuntimeIsConnected(formState.serverId ?? "");
 
   const availableProvidersQuery = useQuery({
     queryKey: ["availableProviders", formState.serverId],