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/mcp-server/tests/interrupt.rs

@@ -81,6 +81,7 @@ async fn shell_command_interruption() -> anyhow::Result<()> {
             sandbox: None,
             config: None,
             base_instructions: None,
+            include_plan_tool: None,
         })
         .await?;