Open umbrella PR for May 21, 2026 pricing + packaging launch

Umbrella tracker for the May 21, 2026 pricing and packaging docs
launch. Populates as the three thematic PRs land:

- #114 - Credits, billing, and cloud-agent team billing
- #115 - BYOK + custom inference endpoint + BYOLLM
- #116 - Plans summary, pricing FAQs, teams copy

Co-Authored-By: Oz <oz-agent@warp.dev>

Author: hongyi-chen