feat: add persistent appliance hostname and Bash prompt

[castrojo]

What to build

Give an installed Bluefin Server appliance a durable operator-selected hostname. When no hostname is provisioned, use blueserver-0; recommend the blueserver-N sequence for later appliances. Persist the selected hostname across OS DDI updates, validate it before applying it, and make interactive Bash show username@hostname with only the @ in blue.

Acceptance criteria

• An unconfigured installed appliance uses blueserver-0 as its static hostname.
• Provisioning can supply a valid selected hostname before first boot; invalid values fail visibly without silently selecting another name.
• The hostname survives reboot and OS DDI replacement, and an authenticated operator can rename it persistently with hostnamectl.
• Interactive Bash renders the username and current hostname with a blue @, retaining directory context and correct $/# privilege markers.
• Focused tests cover defaulting, validation, persistence, and the prompt contract.

Blocked by

None (can start immediately).

Design

docs/superpowers/specs/2026-09-09-appliance-identity-design.md