feat(agents): add Abacus.AI CLI

Follow-up to https://github.com/vercel-labs/skills/pull/2415#issuecomment-6756743346.

Abacus.AI CLI reads skills from .agents/skills and ~/.agents/skills, so this only lists it by name: an abacusai entry in src/agents.ts (detected through ~/.abacusai), plus the regenerated docs and keywords.