Add shared App Server live-peer mode

[moshthepitt]

Why

ACP client should join Codex thread already open in stock TUI.

stock Codex TUI
        |
same App Server + same thread
        |
ACP client

No shadow session. No TUI scraping. Codex owns transcript.

What

• CODEX_APP_SERVER_URL attaches to Unix socket or WebSocket.
• Opt-in _meta.codex.livePeer.version = 1.
• Keep event, approval, elicitation handlers alive outside ACP-owned prompts.
• Emit terminal user messages and exact turn start/finish events.
• Pass clientUserMessageId through prompt and steering; echo it back.
• Frame authoritative history replay by turn.
• Report active turn on load/resume.
• Closing ACP closes its socket. Never shared App Server.

Capability advertisement:

{
  "codex": {
    "livePeer": {
      "version": 1,
      "ambientEvents": true,
      "interactions": true,
      "userMessages": true,
      "clientUserMessageIds": true,
      "turnLifecycle": true,
      "historyReplay": true
    }
  }
}

No live-peer request: old behavior.

Why replay framing

Transport reconnect alone lies. Client can miss completed turn.

Now load emits:

history started
  turn started
  items
  turn finished
history finished

Client can baseline old history, reconcile missed turns, dedupe by native IDs.

Proof

• Full suite: 340 passed, 28 skipped.
• Typecheck: pass.
• Build: pass.
• Stock Codex CLI 0.146.0:
  • bare TUI and ACP share one thread;
  • both directions render;
  • steering works;
  • approvals still work;
  • history load and runtime capabilities pass.

No Codex version pin. Client gates on negotiated capability.