buzz-acp: add a Codex project mode for managed-agent sessions

## Problem

Buzz keeps the right context boundary today: one ACP session per agent and channel. Codex Desktop nevertheless lists those externally triggered sessions alongside the operator's own work. Busy Buzz channels therefore crowd the normal Codex inbox.

## Proposed first implementation

Add an optional **Codex project mode** for Codex-backed managed agents.

- Configure an existing Codex project for the agent.
- When `codex-acp` starts a new Codex thread, pass that project identifier to Codex.
- Keep Buzz' existing `channel_id -> session_id` lifecycle unchanged.
- Keep non-Codex ACP agents unchanged.

Codex then groups the persistent Buzz sessions in the selected project instead of leaving them in the normal inbox.

## Acceptance criteria

- Two Buzz channels for the same Codex-backed agent create or resume separate channel sessions inside the configured Codex project.
- Follow-up messages in a channel retain the existing agent context.
- Without a configured project, current behavior remains unchanged.
- Verify the result against the supported Codex Desktop/App Server version because project APIs are experimental.

## Out of scope

A generic ACP `hidden` or `ephemeral` switch. ACP does not standardize either option, and ephemeral Codex threads would lose the desired follow-up context. A future hidden persistent mode needs Codex or `codex-acp` support.

## Related

#5439 controls reply placement in Buzz. This issue concerns the Codex Desktop presentation of persistent `buzz-acp` sessions.