fix(sandbox): rotate openshell.log daily, keep 3 files

[Kh4L]

Summary

• Replaces the unbounded append-only /var/log/openshell.log with a RollingFileAppender that rotates daily and retains at most 3 files
• No new dependencies — tracing-appender is already a workspace dependency in openshell-sandbox
• Falls back to stdout-only logging if /var/log is not writable (existing behavior preserved)

Related Issue

Closes #30

Changes

• crates/openshell-sandbox/src/main.rs: replace std::fs::OpenOptions + non_blocking(file) with RollingFileAppender::builder() configured for daily rotation and a 3-file cap

Testing

• mise run test passes
• Sandbox logs appear as /var/log/openshell.YYYY-MM-DD.log inside a running sandbox
• Old log files beyond the 3-file limit are pruned on rotation

Checklist

• Follows Conventional Commits format
• Signed-off-by line included (git commit -s)
• No new dependencies introduced

[github-actions]

Thank you for your interest in contributing to OpenShell, @Kh4L.

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.

[github-actions]

All contributors have signed the DCO ✍️ ✅
_{Posted by the DCO Assistant Lite bot.}

[Kh4L]

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

[Kh4L]

recheck