Moves the 35 @deprecated markers from 3.7.0 (already shipped with them in
place) to 3.8.0, and adds scripts/plugin_api_usage.py plus the generated
docs/DEPRECATIONS_3.8.md: who still calls or overrides each deprecated
method across core, the monorepo and third-party plugins. Removes nothing.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
