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/user/mod.rs

@@ -49,7 +49,6 @@ struct ContextInfo {
     available_models: Vec<String>,
     prompt_file: String,
     backend: String,
-    system_prompt_chars: usize,
     context_message_chars: usize,
 }
 
@@ -519,10 +518,6 @@ pub struct CliArgs {
     #[arg(long)]
     pub show_config: bool,
 
-    /// Override all prompt assembly with this file
-    #[arg(long)]
-    pub system_prompt_file: Option<PathBuf>,
-
     /// Project memory directory
     #[arg(long)]
     pub memory_project: Option<PathBuf>,