Drop redundant system prompt — all info is in memory nodes

The system prompt duplicated what's already in core-personality and
other memory nodes. Moving everything to memory means it's all
trainable data rather than hardcoded strings.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/oneshot.rs

@@ -258,12 +258,12 @@ impl AutoAgent {
         let cli = crate::user::CliArgs::default();
         let (app, _) = crate::config::load_app(&cli)
             .map_err(|e| format!("config: {}", e))?;
-        let (system_prompt, personality) = crate::config::reload_for_model(
+        let personality = crate::config::reload_for_model(
             &app, &app.prompts.other,
         ).map_err(|e| format!("config: {}", e))?;
 
         let agent = Agent::new(
-            client, system_prompt, personality,
+            client, personality,
             app, String::new(),
             None,
             super::tools::ActiveTools::new(),