feat(tui): warn on bare null sections in config.yaml

Tolerating null top-level keys silently drops user settings (e.g.
`agent.system_prompt` next to a bare `agent:` line is gone). Probe at
session create, log via `logger.warning`, and surface in the boot info
under `config_warning` — rendered in the TUI feed alongside the existing
`credential_warning` banner.

ui-tui/src/app/useSessionLifecycle.ts

@@ -142,6 +142,10 @@ export function useSessionLifecycle(opts: UseSessionLifecycleOptions) {
         sys(`warning: ${info.credential_warning}`)
       }
 
+      if (info?.config_warning) {
+        sys(`warning: ${info.config_warning}`)
+      }
+
       if (msg) {
         sys(msg)
       }