#18570: chore: scaffold prompt compression proposal skeleton

## Summary\n- add proposal skeleton for optional prompt compression\n- add module/test directory skeletons\n- no runtime behavior changes\n\n## Why\nPrepare a clean upstream path for feature-flagged compression integration without monkey-patches.\n\n## Follow-ups\n- integrate feature flag in config schema\n- wire prompt pipeline fallback-safe compression\n- add unit/integration tests\n