chore: remove responses-api-proxy from the multitool (#4404)

This removes the `codex responses-api-proxy` subcommand in favor of
running it as a standalone CLI.

As part of this change, we:

- remove the dependency on `tokio`/`async/await` as well as `codex_arg0`
- introduce the use of `pre_main_hardening()` so `CODEX_SECURE_MODE=1`
is not required

---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/4404).
* #4406
* __->__ #4404
* #4403

codex-rs/responses-api-proxy/src/main.rs

@@ -1,14 +1,12 @@
-use anyhow::Context;
 use clap::Parser;
-use codex_arg0::arg0_dispatch_or_else;
 use codex_responses_api_proxy::Args as ResponsesApiProxyArgs;
 
-pub fn main() -> anyhow::Result<()> {
-    arg0_dispatch_or_else(|_codex_linux_sandbox_exe| async move {
-        let args = ResponsesApiProxyArgs::parse();
-        tokio::task::spawn_blocking(move || codex_responses_api_proxy::run_main(args))
-            .await
-            .context("responses-api-proxy blocking task panicked")??;
-        Ok(())
-    })
+#[ctor::ctor]
+fn pre_main() {
+    codex_process_hardening::pre_main_hardening();
+}
+
+pub fn main() -> anyhow::Result<()> {
+    let args = ResponsesApiProxyArgs::parse();
+    codex_responses_api_proxy::run_main(args)
 }