fix: introduce codex-protocol crate (#2355)

codex-rs/protocol/src/message_history.rs

@@ -0,0 +1,9 @@
+use serde::Deserialize;
+use serde::Serialize;
+
+#[derive(Serialize, Deserialize, Debug, Clone)]
+pub struct HistoryEntry {
+    pub session_id: String,
+    pub ts: u64,
+    pub text: String,
+}