feat(hosts): expand agent host coverage

[samzong]

What's changed?

• add host adapters for Antigravity 2.0, Grok Build, Kimchi, MiniMax Code, Posit Assistant, and ZCode
• allow dotted ecosystem aliases and explicit-only hosts with empty detection paths
• refresh golden baselines, release smoke assertions, and generated SDK host specs

Why

• align kitup's host registry with current npx skills and gh skill coverage without importing unsupported detection mechanisms

Verification

• make check
• git diff --cached --check