Forget semantics: cascade, tombstone vs hard delete

[antejavor]

Part of #374

Question

What does "forget" mean for a session, a memory and an entity, and is it a hard delete or a tombstone?

Sub-questions

• Session forget cascade. Actions, Agents, Episode, Chunks: clear. What about entities and relations that this session supports together with others? Is support counted per HAS_CHUNK provenance, and does an entity disappear only when it has zero remaining support?
• Tombstone vs hard delete. A tombstone stops re-reconciliation or a re-import from resurrecting forgotten content. A hard delete is what a privacy request actually requires. Can both be true: hard-delete the content, keep a content-free tombstone keyed by session_id / hash?
• Interaction with Grilling: retention/compression policy for raw collection-tier data #267 retention. Retention deletes Collection-Tier data after builders consume it, while forget deletes everything derived. Is one mechanism the general case of the other?
• Memory forget. delete_memory exists. Does forgetting a Memory also retract entities extracted from it?