feat: add spec-kit-onboard extension to community catalog (#1991)

Adds the onboard extension (v2.1.0) — contextual onboarding and
progressive growth for developers new to spec-kit projects.

- 7 commands: start, explain, trail, quiz, badge, mentor, team
- 3 hooks: after-implement, before-implement, after-explain
- Repository: https://github.com/dmux/spec-kit-onboard