uds: add async Unix socket crate (#18254)

## Summary
- add a codex-uds crate with async UnixListener and UnixStream wrappers
- expose helpers for private socket directory setup and stale socket
path checks
- migrate codex-stdio-to-uds onto codex-uds and Tokio-based stdio/socket
relaying
- update the CLI stdio-to-uds command path for the async runner

## Tests
- cargo test -p codex-uds -p codex-stdio-to-uds
- cargo test -p codex-cli
- just fmt
- just fix -p codex-uds
- just fix -p codex-stdio-to-uds
- just fix -p codex-cli
- just bazel-lock-check
- git diff --check

codex-rs/cli/src/main.rs

@@ -1090,8 +1090,7 @@ async fn cli_main(arg0_paths: Arg0DispatchPaths) -> anyhow::Result<()> {
                 "stdio-to-uds",
             )?;
             let socket_path = cmd.socket_path;
-            tokio::task::spawn_blocking(move || codex_stdio_to_uds::run(socket_path.as_path()))
-                .await??;
+            codex_stdio_to_uds::run(socket_path.as_path()).await?;
         }
         Some(Subcommand::ExecServer(cmd)) => {
             reject_remote_mode_for_subcommand(