Allow a way to attach a system prompt to the ACP server

### Describe the feature or problem you'd like to solve

Currently, the ACP mode does not provide a way to supply a system prompt.

### Proposed solution

In some examples like https://github.com/zed-industries/claude-agent-acp/pull/91, the `_meta` field is used to supply a system prompt. Without this capability, we can only edit the agent files to supply a system prompt, and that file is static with the agent.

### Example prompts or workflows

_No response_

### Additional context

_No response_