ACP (Agent Client Protocol) support for ZCode

ZCode is a great harness, but it can currently only be driven through the desktop app or the undocumented zcode app-server protocol. Orchestration tools and editors (Zed, Neovim, JetBrains, agent frameworks) standardize on the Agent Client Protocol — Claude Code, Codex, Gemini CLI, Cursor, OpenCode, and GitHub Copilot all ship ACP implementations, and it's the reason they can be composed programmatically.

Today, programmatic ZCode use is blocked by:

1. No ACP surface in the CLI.
2. The app-server protocol being undocumented, with version skew between the front-end and the agent bundle.
3. OAuth credentials being device-encrypted, so headless/CI/sandbox runs cannot reuse a desktop login (an explicit API-key provider config is required).

Request: an officially supported ACP mode for ZCode (even experimental, e.g. zcode --acp), registered in the ACP registry. Community interest exists — third parties have already built GLM ACP agents against the Coding Plan endpoint to fill this gap. Happy to test pre-releases.

Start by reviewing the CLI entry points for the undocumented zcode app-server protocol and the existing OAuth credential handling. Compare the required ACP surface with the ACP registry, including headless API-key configuration and front-end/agent-bundle versioning. Done means an officially supported, testable ACP mode such as zcode --acp is available for orchestration tools.