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/gatewayTypes.ts

@@ -93,7 +93,7 @@ export interface SetupStatusResponse {
 // ── Session lifecycle ────────────────────────────────────────────────
 
 export interface SessionCreateResponse {
-  info?: SessionInfo & { credential_warning?: string }
+  info?: SessionInfo & { config_warning?: string; credential_warning?: string }
   session_id: string
 }