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/mind/mod.rs

@@ -292,7 +292,6 @@ impl Mind {
 
         let agent = Agent::new(
             client,
-            config.system_prompt.clone(),
             config.context_parts.clone(),
             config.app.clone(),
             config.prompt_file.clone(),