Add exec-server stub server and protocol docs (#15089)

Stacked PR 1/3.

This is the initialize-only exec-server stub slice: binary/client
scaffolding and protocol docs, without exec/filesystem implementation.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/exec-server/src/server.rs

@@ -0,0 +1,18 @@
+mod handler;
+mod jsonrpc;
+mod processor;
+mod transport;
+
+pub(crate) use handler::ExecServerHandler;
+pub use transport::ExecServerTransport;
+pub use transport::ExecServerTransportParseError;
+
+pub async fn run_main() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+    run_main_with_transport(ExecServerTransport::Stdio).await
+}
+
+pub async fn run_main_with_transport(
+    transport: ExecServerTransport,
+) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+    transport::run_transport(transport).await
+}