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/src/codex_tool_runner.rs

@@ -263,6 +263,7 @@ async fn run_codex_tool_session_inner(
                     | EventMsg::PatchApplyBegin(_)
                     | EventMsg::PatchApplyEnd(_)
                     | EventMsg::GetHistoryEntryResponse(_)
+                    | EventMsg::PlanUpdate(_)
                     | EventMsg::ShutdownComplete => {
                         // For now, we do not do anything extra for these
                         // events. Note that