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

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[//]: # (BEGIN SAPLING FOOTER)
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* #4406
* __->__ #4404
* #4403

codex-rs/Cargo.toml

@@ -53,7 +53,6 @@ codex-ollama = { path = "ollama" }
 codex-process-hardening = { path = "process-hardening" }
 codex-protocol = { path = "protocol" }
 codex-protocol-ts = { path = "protocol-ts" }
-codex-responses-api-proxy = { path = "responses-api-proxy" }
 codex-rmcp-client = { path = "rmcp-client" }
 codex-tui = { path = "tui" }
 codex-utils-readiness = { path = "utils/readiness" }