The generic "node command not allowed" error gives no indication of why the
command was rejected, making it hard to diagnose issues (e.g. running
`nodes notify` against a Linux node that does not declare `system.notify`).
Include the rejection reason and node platform in the error message so
callers can tell whether the command is not supported by the node, not in
the platform allowlist, or the node did not advertise its capabilities.
Fixes#24616
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
(cherry picked from commit e3d74619bc)
