docs/mode-economics.md: stale token table — lists future skills that exist, missing shipped skills

[rbowen]

What should happen

1. Remove "future skill" label from pairing-multi-agent-review (it's shipped).
2. Add pre-first-pr-check and the 4 Mentoring-family skills missing from the token table.

Why

The economics doc is the reference for adopters estimating token costs. Missing skills and stale "future" markers make cost planning unreliable.

Which layer

Documentation (docs/, README.md, CONTRIBUTING.md)