`exec` without explicit VM ID fails to resolve HEAD

[AlephNotation]

Bug

When using vers exec without an explicit VM ID (relying on HEAD), the first argument of the command gets parsed as a VM ID, causing a UUID parse error.

Repro

$ vers head
7ab376e8-9c15-4109-abd2-e14abe460429

$ vers exec echo hello
Error: exec: API error 400: Invalid URL: Cannot parse `vm_id` with value `echo`: UUID parsing failed...

$ vers exec -- echo hello    # also fails the same way

$ vers execute 7ab376e8-9c15-4109-abd2-e14abe460429 echo hello   # works
hello

Expected

When HEAD is set, vers exec <command> [args...] should use the HEAD VM, same as pause, resume, info, etc. The CLI should check whether the first positional arg is a valid UUID before treating it as a VM ID.

Impact

This is especially painful for agents/scripts — every exec call requires a preceding vers head call to get the VM ID, defeating the purpose of HEAD.