docs: make release-only a first-class adoption path

[joshua-temple]

Problem

The Adoption Guide was structured almost entirely around multi-environment pipelines. Release-only (no-env) repos were technically mentioned but marginalized: a one-line footnote at the bottom, with prose elsewhere quietly assuming every adopter has environments.

Fix

Make release-only a first-class path in adoption.md, without overclaiming:

• Add an upfront framing that cascade fits two shapes equally (deploy-through-environments and release-only).
• Scope the environments prerequisite to the deploy case; a release-only repo skips it.
• Add a brief release-only note in the env-choosing step.
• Promote the no-env topology bullet from a library/CLI footnote to a first-class shape, broadening the fit (libraries, CLIs, SDKs, published images, schemas/packages).
• Preserve the real constraint: hotfix and rollback need two or more environments, so they do not apply to release-only repos.

Verification

• Docs-only change to one Markdown file.
• Independent review: APPROVE. Claims verified against code (no-env preset is real; release-only still gets version derivation, changelog, and release management; hotfix/rollback 2+ env requirement preserved).
• In-page anchor and cross-page links checked.