fix(cli): write exec stderr events to stderr in interactive mode

[andrewwhitecdw]

Summary

In interactive openshell exec, stderr payloads from the sandbox were written to stdout because the Stderr arm of the event loop was a verbatim copy of the Stdout arm (stdout.lock()). The non-interactive exec path in the same file handles the identical event with stderr.lock(). This broke shell redirection (e.g. openshell exec ... 2>err.txt) and piping of stdout in interactive mode.

Related Issue

N/A — small copy-paste fix found during code review.

Changes

• Bind stderr in the interactive exec event loop and lock it in the Stderr arm, matching the non-interactive path

Testing

• mise run pre-commit passes (mise unavailable in this environment; ran equivalent cargo fmt + cargo clippy -p openshell-cli --all-targets — clean)
• Unit tests added/updated (event loop requires a live gRPC stream; no existing test seam for this path)
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.

[github-actions]

Thank you for your submission! We ask that you sign our Developer Certificate of Origin before we can accept your contribution. You can sign the DCO by adding a comment below using this text:

---

I have read the DCO document and I hereby sign the DCO.

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the DCO Assistant Lite bot.}

[github-actions]

Thank you for your interest in contributing to OpenShell, @andrewwhitecdw.

This project uses a vouch system for first-time contributors. Before submitting a pull request, you need to be vouched by a maintainer.

To get vouched:

1. Open a Vouch Request discussion.
2. Describe what you want to change and why.
3. Write in your own words — do not have an AI generate the request.
4. A maintainer will comment /vouch if approved.
5. Once vouched, open a new PR (preferred) or reopen this one after a few minutes.

See CONTRIBUTING.md for details.