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/exec/src/event_processor_with_human_output.rs

@@ -1,5 +1,6 @@
 use codex_common::elapsed::format_elapsed;
 use codex_core::config::Config;
+use codex_core::plan_tool::UpdatePlanArgs;
 use codex_core::protocol::AgentMessageDeltaEvent;
 use codex_core::protocol::AgentMessageEvent;
 use codex_core::protocol::AgentReasoningDeltaEvent;
@@ -513,6 +514,11 @@ impl EventProcessor for EventProcessorWithHumanOutput {
                 ts_println!(self, "model: {}", model);
                 println!();
             }
+            EventMsg::PlanUpdate(plan_update_event) => {
+                let UpdatePlanArgs { explanation, plan } = plan_update_event;
+                ts_println!(self, "explanation: {explanation:?}");
+                ts_println!(self, "plan: {plan:?}");
+            }
             EventMsg::GetHistoryEntryResponse(_) => {
                 // Currently ignored in exec output.
             }

codex-rs/exec/src/lib.rs

@@ -126,6 +126,7 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         model_provider: None,
         codex_linux_sandbox_exe,
         base_instructions: None,
+        include_plan_tool: None,
     };
     // Parse `-c` overrides.
     let cli_kv_overrides = match config_overrides.parse_overrides() {