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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* #4406
* __->__ #4404
* #4403

codex-rs/Cargo.lock

@@ -695,7 +695,6 @@ dependencies = [
  "codex-process-hardening",
  "codex-protocol",
  "codex-protocol-ts",
- "codex-responses-api-proxy",
  "codex-tui",
  "ctor 0.5.0",
  "owo-colors",
@@ -996,13 +995,13 @@ version = "0.0.0"
 dependencies = [
  "anyhow",
  "clap",
- "codex-arg0",
+ "codex-process-hardening",
+ "ctor 0.5.0",
  "libc",
  "reqwest",
  "serde",
  "serde_json",
  "tiny_http",
- "tokio",
  "zeroize",
 ]