Per-session serialization

The hub is the single writer per session file and serialises writers in process.

Context

Several agents can share one named session and write to the same brain. Optimistic locking with a version check in SQL would need a version column the AgentFS store does not have, and the hub is a single process, so a cross-process lock is not needed either.

Decision

The hub process is the single writer for each session file, so it serialises writers with a per-session in-process lock. Multiple agents in one session write through the hub and are ordered by that lock. No optimistic locking in v1.

Consequences

• Writers to distinct sessions never block each other; writers to one session serialise.
• A version column is reserved in the schema for a future multi-process model, but nothing depends on it.
• Cross-session write conflicts remain structurally impossible.