[mcp-server] Add reply tool call (#1643)

## Summary
Adds a new mcp tool call, `codex-reply`, so we can continue existing
sessions. This is a first draft and does not yet support sessions from
previous processes.

## Testing
- [x] tested with mcp client

codex-rs/exec/src/lib.rs

@@ -153,7 +153,7 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         .with_writer(std::io::stderr)
         .try_init();
 
-    let (codex_wrapper, event, ctrl_c) = codex_wrapper::init_codex(config).await?;
+    let (codex_wrapper, event, ctrl_c, _session_id) = codex_wrapper::init_codex(config).await?;
     let codex = Arc::new(codex_wrapper);
     info!("Codex initialized with event: {event:?}");