fork conversation from a previous message (#2575)

This can be the underlying logic in order to start a conversation from a
previous message. will need some love in the UI.

Base for building this: #2588

codex-rs/exec/src/event_processor_with_human_output.rs

@@ -540,6 +540,7 @@ impl EventProcessor for EventProcessorWithHumanOutput {
                 }
             },
             EventMsg::ShutdownComplete => return CodexStatus::Shutdown,
+            EventMsg::ConversationHistory(_) => {}
         }
         CodexStatus::Running
     }