feat: gate reboot decisions on @rebootable_seed_types module attribute
Replace hardcoded "nixos" checks in maybe_reboot/3 and reboot_reason/3
with a @rebootable_seed_types module attribute, allowing reboot logic
to be skipped entirely for non-rebootable seed types.
sow-128
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>