feat: enhance safety and crash resilience in file operations; add fsync to critical writes and checks for path traversal

src/app/subagent/engine.rs

@@ -101,9 +101,13 @@ pub fn run_subagent(ctx: SubagentContext, tx: mpsc::Sender<SubagentEvent>) -> an
     let (tools, tdefs) = build_subagent_tools(&ctx.allowed_tools);
     let tdefs_opt: Option<Vec<ToolDef>> = if tdefs.is_empty() { None } else { Some(tdefs) };
 
+    // Cache provider config once before the loop instead of re-resolving
+    // from disk on every step (Settings::load + AppConfig::load each parse
+    // JSON files, and the config cannot change between steps).
+    let (api_key, model, base_url) = resolve_provider_config();
+    let client = crate::service::provider::LlmClient::new(api_key, model, base_url);
+
     for step in 0..ctx.max_steps {
-        let (api_key, model, base_url) = resolve_provider_config();
-        let client = crate::service::provider::LlmClient::new(api_key, model, base_url);
 
         // Use the structured tool-calling API so the LLM can request tools with
         // proper arguments, exactly like the main agent does.