DL-428

Server mechanism for DL-426: recompute SHA-256 and reject a mismatch, return early on this session’s own index row, else PUT then insert into agent_session_blobs (session_id, sha256, size_bytes; new migration); reads are keyed by (session, hash) and need that session’s row; PutSessionBlob never probes object existence; no object store is CodeFailedPrecondition

Status: Active (Matt, 2026-10-06)