Layer the filesystem RPC implementation and exec-server-only client cleanup on top of the process RPC follow-up.\n\nCo-authored-by: Codex <noreply@openai.com>
Keep only the standalone handshake transport in the first PR. Leave exec RPC behavior unimplemented here so the working process implementation can land in a separate follow-up.\n\nCo-authored-by: Codex <noreply@openai.com>
This adds the standalone exec-server stdio JSON-RPC crate and its
smoke tests without wiring it into the CLI or unified-exec yet.
Co-authored-by: Codex <noreply@openai.com>
