docs: document hotfix deploy as a placeholder; roll the changelog to 0.16.1

[joshua-temple]

Two doc-truth fixes.

• Hotfix deploy honesty: the generated cascade-hotfix workflow's deploy/rollback jobs are echo-only placeholders (they hold the environment gate and print, but never invoke the configured deploy workflow). Docs previously claimed hotfix deploys the fix. Corrected across guides/hotfix, reference/generated-workflows, reference/cli, start/how-it-works, and README to state actual behavior (cherry-pick/build/tag/release are real; deploy is a manual follow-up). The standalone cascade-rollback workflow uses real reusable-workflow calls and is noted as unaffected.
• CHANGELOG: rolled the v0.16.1 entries out of [Unreleased] into a dated [0.16.1] section with compare links.

Verification: README schema test passes; docs site builds on Node 22 (CI docs-build gate).