Add LLM system prompt override

Allow overriding the LLM system prompt via the `LLM_SYSTEM_PROMPT`
environment variable. This provides flexibility for customizing LLM
behavior without code changes. A default prompt is used if the
environment variable is unset or empty.

server/src/main.rs

@@ -48,6 +48,17 @@ async fn main() {
         );
     }
 
+    {
+        let custom = std::env::var("LLM_SYSTEM_PROMPT")
+            .map(|v| !v.is_empty())
+            .unwrap_or(false);
+        info!(
+            prompt_source = if custom { "env" } else { "default" },
+            prompt_chars = config.llm_system_prompt.chars().count(),
+            "system prompt loaded"
+        );
+    }
+
     // Shared HTTP client for both downstream pipelines.
     let http_client = reqwest::Client::builder()
         .build()