vpctl 0.6.0

Version compatibility check: `release generate` and `secret generate` now verify that vpctl satisfies the minimum version required by the release package ( `compatibility.yaml` ). The command blocks execution and displays a clear upgrade message when vpctl is too old. Use `--skip-version-check` to bypass. Dev builds and RC versions are handled gracefully.

`image sync --concurrency` flag: processes multiple images in parallel using a worker pool (default: 1 = sequential).

`vpctl version` no longer prints an irrelevant manifest-not-found warning.