Add an experimental plan tool (#1726)

This adds a tool the model can call to update a plan. The tool doesn't
actually _do_ anything but it gives clients a chance to read and render
the structured plan. We will likely iterate on the prompt and tools
exposed for planning over time.

codex-rs/core/src/codex.rs

@@ -55,6 +55,7 @@ use crate::models::ReasoningItemReasoningSummary;
 use crate::models::ResponseInputItem;
 use crate::models::ResponseItem;
 use crate::models::ShellToolCallParams;
+use crate::plan_tool::handle_update_plan;
 use crate::project_doc::get_user_instructions;
 use crate::protocol::AgentMessageDeltaEvent;
 use crate::protocol::AgentMessageEvent;
@@ -1336,6 +1337,7 @@ async fn handle_function_call(
             };
             handle_container_exec_with_params(params, sess, sub_id, call_id).await
         }
+        "update_plan" => handle_update_plan(sess, arguments, sub_id, call_id).await,
         _ => {
             match sess.mcp_connection_manager.parse_tool_name(&name) {
                 Some((server, tool_name)) => {