Add session-meta parameter for system prompt customization

[ElleNajt]

Summary

Add :session-meta parameter to agent-shell-start and related functions, allowing callers to pass metadata when creating ACP sessions. This enables customizing the system prompt by appending to the default, which is useful for multi-agent systems where certain agents need persistent instructions that survive context compaction.

Changes

• Added :session-meta parameter to agent-shell--make-state
• Added :session-meta parameter to agent-shell-start and agent-shell--start
• Store session-meta in buffer-local state
• Pass it to acp-make-session-new-request when creating the session

Example Usage

(agent-shell-start
  :config (agent-shell--resolve-preferred-config)
  :session-meta '((systemPrompt . ((append . "Custom instructions...")))))

Or set it on an existing buffer before the first prompt:

(map-put! agent-shell--state :session-meta
          '((systemPrompt . ((append . "Instructions here")))))

Use Case

This is useful for building systems like meta-agent-shell where "dispatcher" agents need persistent instructions. Previously, instructions sent as user messages would be lost on context compaction. With session-meta, they're appended to the system prompt and persist.

Dependencies

Requires acp.el with :meta parameter support (merged in xenodium/acp.el#13).

[xenodium]

I'm wondering how to scale this a bit better, so we don't keep injecting parameters every time we watn to customize a request. I'm thinking perhaps we can use request-decorating functions that receive and return a request, giving you the opportunity to customize the request before it's sent out. Lemme think a bit more about this. Would a solution along these lines work for your purpose?

[ElleNajt]

Yeah that would be good! Mostly I just want to be able to pass other parameters to claude-code-acp.