ACP server support

I am currently trying to find a sandboxed solution to setup OpenCode using local models in a way that integrates with Zed.

It would be nice to be able to start the ACP server for any supported coding agent, instead of the agent's CLI/TUI. This way, integrating with Zed would only require configuring a clampdown command that would start an ACP server directly from the IDE. Then the UI would be handled by Zed.

To illustrate, the Zed config would look something like this:

```json
"agent_servers": {
"opencode": {
"type": "custom",
"command": "clampdown",
"args": ["opencode", "acp"],
"env": {}
},
```

Where `acp` is already a subcommand that exists for `opencode`.