feat: centralize default constants and refactor overlay enter handling in TUI

apps/infrastructure/src/subagent/provider.rs

@@ -67,7 +67,7 @@ impl SubagentProvider {
 ///
 /// Flow: reads `settings.provider` and `settings.model` → if model is empty,
 /// falls back to the provider config's `default_model` → if that is also
-/// empty, uses `"deepseek-v4-flash-free"` as the ultimate default.
+/// empty, uses the domain default model constant.
 #[instrument]
 pub fn resolve_subagent_provider(
     settings: &zesdex_domain::cms::Settings,
@@ -82,7 +82,7 @@ pub fn resolve_subagent_provider(
             .providers
             .get(&provider)
             .and_then(|p| p.default_model.clone())
-            .unwrap_or_else(|| "deepseek-v4-flash-free".to_string())
+            .unwrap_or_else(|| zesdex_domain::agent::defaults::DEFAULT_MODEL.to_string())
     } else {
         model
     };