feat: compaction prompt configurable (#5959)

```
 codex -c compact_prompt="Summarize in bullet points"
 ```

codex-rs/core/src/codex/compact.rs

@@ -39,9 +39,8 @@ pub(crate) async fn run_inline_auto_compact_task(
     sess: Arc<Session>,
     turn_context: Arc<TurnContext>,
 ) {
-    let input = vec![UserInput::Text {
-        text: SUMMARIZATION_PROMPT.to_string(),
-    }];
+    let prompt = turn_context.compact_prompt().to_string();
+    let input = vec![UserInput::Text { text: prompt }];
     run_compact_task_inner(sess, turn_context, input).await;
 }