Experimental Codex run backend via app-server JSON-RPC (daemon thread + turn)

[devgony]

Context

Recent Codex CLIs (v0.149+; verified locally on 0.153.0-alpha.5) ship an experimental app-server daemon: codex app-server daemon start|stop|..., codex agents (browse agent sessions on the shared daemon), codex queue --thread <uuid> --message, WebSocket/unix-socket remote control with bearer auth, and protocol schema generators (codex app-server generate-json-schema).

However codex exec — what rein run uses for the Codex backend — is still a standalone foreground process:

• no claude --bg-style "submit a background run to the daemon" CLI entry point,
• exec sessions do not register with the daemon (codex agents won't list them),
• exec/CLI sessions don't appear in the Codex desktop app (tracked upstream: Make Codex CLI sessions available in Codex desktop history openai/codex#21079, Feature request: show local Codex CLI sessions in the desktop app openai/codex#24197, Codex Desktop App does not display existing local sessions from shared CODEX_HOME openai/codex#14389, ACP-spawned sessions don't appear in the Codex desktop app openai/codex#16385 — the app reads its own state_5.sqlite index, not ~/.codex/sessions rollouts).

So rein currently backgrounds codex exec itself and owns the log/status files (see the DEFAULT_CODEX_RUN_CMD comment in src/commands/exec.rs).

Proposal (experimental, opt-in)

Add a third Codex launch mode that talks JSON-RPC to the local app-server daemon instead of spawning codex exec:

1. Ensure the daemon is up (codex app-server daemon start, or connect to the control socket at $CODEX_HOME/app-server-control/app-server-control.sock).
2. Create a thread with the task worktree as cwd, run a turn with the rein item/task prompt ($REIN_PROMPT).
3. Record the thread id in the task's RunRecord (same shape as today — session, agent: "codex", item), stream turn events into <store>/runs/*.log so L / rein logs keep working, and reuse the existing post-run rein todo completeness check.
4. a (attach) keeps resuming by thread id; sessions launched this way should also show up in codex agents.

Config: rein.runAgent = codex-app-server (or a rein.codexDaemon = true toggle). Keep codex exec as the default.

Why not now

• The app-server protocol and daemon are flagged [experimental] and churn between alphas; the daemon also requires the managed standalone Codex install (~/.codex/packages/standalone/current/codex).
• Desktop-app visibility would not materialize anyway until Make Codex CLI sessions available in Codex desktop history openai/codex#21079 / Feature request: show local Codex CLI sessions in the desktop app openai/codex#24197 land — the app's history index is separate even from the shared CLI daemon.

Trigger to pick this up

• Make Codex CLI sessions available in Codex desktop history openai/codex#21079 (CLI sessions in desktop history) closes, or
• codex exec gains a daemon/--remote submission path, or
• the app-server protocol graduates out of [experimental].