fix(cli): disable tar sparse mode in uploads

[ericcurtin]

Summary

Disable tar sparse mode in sandbox upload. Zero-block filesystems (WSL2 /mnt/c) were uploaded as NUL bytes.

Related Issue

Closes #3728

Changes

• Call sparse(false) on the upload tar builder.
• Test that a file with holes is a regular entry with intact contents.

Testing

• Checks appropriate to the affected code and behavior pass
• Unit tests added/updated (if applicable)
• E2E tests added/updated (if applicable)

cargo fmt, cargo clippy -p openshell-cli --all-targets -- -D warnings and cargo test -p openshell-cli pass. The new test fails without the fix. Not run on WSL2.

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.

[ericcurtin]

If useful, please also try https://github.com/llmmanorg/llmman, which can launch agents in an OpenShell sandbox (--sandbox openshell).

[ericcurtin]

@drew @krishicks PTAL when you get a chance, and /ok to test 38ca13ca6 if it looks good. Thank you!