Add `--format json` to `execute` command

[AlephNotation]

Feature request

Every other command supports --format json for machine-readable output, but execute does not. Currently, stdout/stderr are printed directly with no structured wrapper, making it difficult to programmatically distinguish command output from CLI errors.

Suggestion

$ vers exec <vm-id> --format json echo hello
{
  "stdout": "hello\n",
  "stderr": "",
  "exit_code": 0
}

Impact

Agents and scripts that parse exec output need to reliably separate stdout, stderr, and exit codes. Today this requires heuristics or checking $? separately.