docs: tighten root README prose into bullets and right-size non-env framing

[joshua-temple]

What

• Convert the dense multi-sentence prose blocks in the root README into lead-in + bullet lists (How it works, Cross-repo artifact tracking, Hotfix any environment, Is cascade right for your repo, What adopting looks like, Roadmap to stable). Hero/badges, mermaid diagrams, code fences, and tables are untouched.
• Lightly soften the one discouraging line so a no-deployment repo reads as a supported shape (no-environment mode still gives conventional-commit versioning and releases) rather than being lumped under "overkill." Deliberately a light touch, not a headline.

Verification

• Docs-only; single file (README.md). Rebased onto current main.
• Independent review: APPROVE. All 19 links and inline-code tokens preserved across the bullet conversions; fences balanced; the softened line is accurate and does not imply hotfix/rollback for no-env.