Support custom hostname override during enrollment

[dkontango]

Summary

Currently schmutz-join always uses the system hostname from os.Hostname(). Some environments (cloud VMs, containers) have unhelpful default hostnames like ip-172-31-42-8 or localhost.

Proposal

Add a --hostname=NAME flag that overrides the detected hostname in the registration request. The controller would use this as the machine's display name and for the SSH service (ssh-<name>.tango).

schmutz-join https://join.example.net --hostname=web-prod-1

Notes

• Flag only — no interactive prompts
• Controller should still record the real hostname in metadata for audit
• Nickname generation stays server-side (unaffected)