Resolves #6 (3mwcn6q5k3w22).
Compaction only changes the model-facing projection. Image payloads remain referenced by durable transcript rows (also used by LCM expansion and forks), so evicting them at compaction would corrupt history. Document existing content-addressed storage and ref-aware reclamation on session deletion; correct an outdated claim that bytes live in transcript rows. No runtime change is needed.
Test reported by implementer: gleam test image_store (107 passed); manager reviewed image store release implementation and documentation.