Hide unreleased network-disk commands from --help

[sammy-vastai]

Summary

Follow-up to #466 (network volumes). Network disks aren't released either — show network-disks and add network-disk were still fully visible in vastai --help and tab completion.

Reuses the same HIDDEN_COMMANDS mechanism from #466: discoverability gate only, not an access gate — both commands still run if typed directly.

Test plan

• poetry run pytest tests/cli — 415 passed
• Manually verified: neither command appears anywhere in vastai --help; both still parse and run when typed directly

[vastzuby]

imo should just remove everything related to network volumes, we can always add it from the history

[sammy-vastai]

imo should just remove everything related to network volumes, we can always add it from the history

Yeah didn't know whether to delete so that's why I tagged some infra/host devs. I think everything network volumes related is now hidden (from --help and autocomplete suggestions). Last pr removed network volume commands and i had missed the 'network disk' related commands. The commands still work if we type them, they're just hidden from users. Should be sufficient, lmk if any concerns