docs(codex-rescue): document the sandbox writable-root constraint (writes outside the repo are silently read as delegation failure)

Summary

When a Claude Code session delegates to the codex-rescue agent and the task specifies an output path outside the repository (e.g. ~/.local/bin/foo), Codex refuses to write:

writing outside of the project; rejected by user approval settings

Codex reports this clearly and even suggests the remedy. The problem is that the constraint is not documented anywhere in the plugin — agents/codex-rescue.md, the skills, and the README contain no mention of writable / sandbox roots.

Why this is costly

The delegating agent typically checks whether the artifact files exist, sees nothing, and concludes "the delegation failed / the result could not be retrieved." It then redoes the work itself. The actual cause — an impossible spec — is only visible in the Codex CLI output, which the parent may never read. This is silent, repeats every session, and wastes both time and tokens.

Reproduction

1. From Claude Code, delegate to codex-rescue with a prompt like: "create ~/.local/bin/mytool and a test script next to it, then run the tests."
2. Codex reports the rejection above; no files are created.
3. Nothing in the plugin documentation would have warned the caller in advance.

Suggested fix

Add a short note to agents/codex-rescue.md (and ideally the runtime/prompting skills), for example:

Sandbox scope: Codex can only write inside the repository and temp directories. Tasks that must produce files elsewhere (~/.local/bin, ~/.config, system paths) should write into the repo and let the caller install them.

Two smaller points that would help the same failure mode:

• Note that runs exceeding the foreground timeout move to the background, and that codex-rescue cannot retrieve its own background result — the caller must read the output file.
• Recommend that the caller read the Codex output rather than inferring success/failure from artifact presence.

Environment

• codex plugin 1.0.6
• Claude Code on macOS (arm64)

Start with agents/codex-rescue.md, then review the related skills and README for existing delegation guidance. Document the writable-root constraint and, if scope permits, the background-timeout and output-reading caveats described in the issue. Done means the plugin documentation warns callers how to handle outputs outside the repository and avoid inferring failure from missing artifacts.