fix(cloudlab): quote ssh arguments in on_nodes (SC2068)

[randomizedcoder]

Bug: unquoted $@ forwarded to ssh in on_nodes (SC2068)

cloudlab/bin/on_nodes runs a caller-supplied command on a range of cluster nodes, forwarding the command and its arguments with a bare, unquoted $@:

for ((i = $first ; i <= $last; i++)); do
    node=node$i
    ...
    ssh -4 $node $@
done

An unquoted $@ is subject to word splitting and glob expansion in on_nodes itself, before ssh ever sees the arguments. So:

• an argument containing spaces ("grep foo bar") is split into several arguments;
• an argument containing a glob metacharacter ('*.c') is expanded against the local filesystem;
• an empty-string argument is dropped entirely.

shellcheck reports this as SC2068 (error).

Fix

Quote both the node and the forwarded arguments so word boundaries reach ssh intact:

-    ssh -4 $node $@
+    ssh -4 "$node" "$@"

Verification (linter gate + bats regression)

Before:  shellcheck --include=SC2068 cloudlab/bin/on_nodes
         line 28: SC2068 (error): Double quote array expansions ...
After:   (clean, exit 0)

A bats scaffold is included as executable documentation (no new CI is wired). It stubs ssh on PATH to record the exact argv it receives, then asserts argument boundaries survive. Run against the old bare-$@ script, three cases fail; after the fix all pass:

$ nix shell nixpkgs#bats -c bats cloudlab/tests/on_nodes.bats
 ✓ usage error when fewer than 3 args
 ✓ simple command is forwarded verbatim
 ✓ argument containing spaces stays a single argument      # RED before fix
 ✓ argument with a glob char is not expanded by on_nodes   # RED before fix
 ✓ empty-string argument is preserved as one argument      # RED before fix

The pre-existing SC2004 style note on the arithmetic loop ($first/$last inside (( ))) is a separate, style-level check and is left untouched to keep this diff focused on the SC2068 defect.