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/protocol/src/protocol.rs

@@ -22,6 +22,7 @@ use uuid::Uuid;
 use crate::config_types::ReasoningEffort as ReasoningEffortConfig;
 use crate::config_types::ReasoningSummary as ReasoningSummaryConfig;
 use crate::message_history::HistoryEntry;
+use crate::models::ResponseItem;
 use crate::parse_command::ParsedCommand;
 use crate::plan_tool::UpdatePlanArgs;
 
@@ -137,6 +138,10 @@ pub enum Op {
     /// Request a single history entry identified by `log_id` + `offset`.
     GetHistoryEntryRequest { offset: usize, log_id: u64 },
 
+    /// Request the full in-memory conversation transcript for the current session.
+    /// Reply is delivered via `EventMsg::ConversationHistory`.
+    GetHistory,
+
     /// Request the list of MCP tools available across all configured servers.
     /// Reply is delivered via `EventMsg::McpListToolsResponse`.
     ListMcpTools,
@@ -471,6 +476,8 @@ pub enum EventMsg {
 
     /// Notification that the agent is shutting down.
     ShutdownComplete,
+
+    ConversationHistory(ConversationHistoryResponseEvent),
 }
 
 // Individual event payload types matching each `EventMsg` variant.
@@ -651,6 +658,14 @@ impl McpToolCallEndEvent {
     }
 }
 
+/// Response payload for `Op::GetHistory` containing the current session's
+/// in-memory transcript.
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct ConversationHistoryResponseEvent {
+    pub conversation_id: Uuid,
+    pub entries: Vec<ResponseItem>,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize)]
 pub struct ExecCommandBeginEvent {
     /// Identifier so this can be paired with the ExecCommandEnd event.