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.

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

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

@@ -0,0 +1,21 @@
+mod client;
+mod client_api;
+mod connection;
+mod local;
+mod protocol;
+mod rpc;
+mod server;
+
+pub use client::ExecServerClient;
+pub use client::ExecServerError;
+pub use client_api::ExecServerClientConnectOptions;
+pub use client_api::RemoteExecServerConnectArgs;
+pub use local::ExecServerLaunchCommand;
+pub use local::SpawnedExecServer;
+pub use local::spawn_local_exec_server;
+pub use protocol::InitializeParams;
+pub use protocol::InitializeResponse;
+pub use server::ExecServerTransport;
+pub use server::ExecServerTransportParseError;
+pub use server::run_main;
+pub use server::run_main_with_transport;