Allow a way to attach a system prompt to the ACP server #1969
Description
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 zed-industries/claude-agent-acp#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