Slack @Codex tasks fail silently before cloud runner starts (no logs), while identical manual Cloud tasks work

Description

Every time I mention @Codex in Slack, the cloud task fails to start: no logs appear at all — no repository checkout, no environment setup, no command execution, no agent activity. The environment Codex selects is the correct, already-existing one for the repository.

Starting a task manually in the ChatGPT/Codex Cloud web UI, using the exact same GitHub repository and the exact same environment, works normally every time.

This isolates the failure to the Slack → Codex Cloud task dispatch path, not to the environment, the repository connection, or the Cloud runner itself.

Steps to reproduce

1. Have a working Codex Cloud environment connected to a GitHub repository (confirmed working via the Codex web UI).
2. Install/connect the Codex Slack app to a workspace and channel.
3. In Slack, mention @Codex with a task referencing that repository (with or without explicitly naming owner/repo and the environment name).
4. Observe: the task is created but never produces any logs or output — it silently fails before the runner starts.
5. Compare: start the identical task directly from Codex Cloud web using the same repo/environment — it runs successfully.

Expected behavior

A Slack-triggered @Codex task should reach the same cloud runner startup and logging behavior as a task created directly in Codex Cloud, given the same repository and environment.

Actual behavior

The Slack-triggered task fails before the runner ever starts. No checkout, setup script execution, or agent progress logs appear anywhere in the task view. This happens consistently, on every attempt, across multiple retries and threads.

Related reports

This looks similar in shape to #1217 and #20093 (environment/task-dispatch failing on the @codex trigger path while manual Cloud execution works fine), though not identical.

Environment

• Trigger: Slack @Codex mention (OpenAI's first-party Codex Slack app)
• Works fine: manual task creation in Codex Cloud web, same repo + environment
• Fails: every Slack-triggered task, no logs produced
• Repro is consistent, not intermittent

The issue names no repository files, tests, or entry points. Start by tracing the Slack @Codex mention through task creation and compare it with the working Codex Cloud web path, using the task view to check whether startup and logging begin. Done means Slack-triggered tasks reach runner startup and show logs for the same repository and environment.