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/tui/src/lib.rs

@@ -79,6 +79,7 @@ pub async fn run_main(
             config_profile: cli.config_profile.clone(),
             codex_linux_sandbox_exe,
             base_instructions: None,
+            include_plan_tool: None,
         };
         // Parse `-c` overrides from the CLI.
         let cli_kv_overrides = match cli.config_overrides.parse_overrides() {