fix: clarify missing openshell CLI prerequisite

[sameertele]

Summary

• add an explicit OpenShell CLI preflight helper for the nemoclaw host-side command path
• fail early with a clear missing-prerequisite message for connect, status, and logs
• document in the command reference that host-side sandbox-management commands require openshell on PATH

Root cause

The current package shells out to openshell for host-side sandbox commands, but a fresh npm install can still reach those commands without any early prerequisite check. That leaves users with a late command not found failure that is easy to misread as a NemoClaw runtime bug.

Validation

• ran npm install in nemoclaw/ to restore the package-local toolchain in this clone
• ran npm run build in nemoclaw/
• PASS

Fixes #410

[miyoungc]

These files are being removed.
#473
#492

Closing this PR.
Thank you for your contribution effort!