Forward-port exec-server and route unified exec through it

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

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

@@ -1,6 +1,7 @@
+mod filesystem;
 mod handler;
-mod jsonrpc;
 mod processor;
+mod registry;
 mod transport;
 
 pub(crate) use handler::ExecServerHandler;