Update a contact memory entry

Create or replace the memory entry with this name; the entry is upserted, so a PUT to a name that does not exist yet creates it. This is a full replace, not a patch: omitting description clears the stored description. Returns { ok: true, entries: <count after the write> }; invalid input returns 400 and an unknown session 404. Staff writes work even when the project's contact-memory feature is disabled.