Adds the agent_sessions table (migration) and a session_store module that
owns its lifecycle:
- ensure_session: idempotently creates the session row on first chat.
- record_turn: accumulates input/output/total tokens, cost and turn count
after each completed turn, bumping updated_at.
- get_session: user-scoped fetch so a tenant can only read its own session.
Backed by SQLite via SQLx, matching the existing service style. Unit-tested
against an in-memory database (idempotency, accumulation, user scoping).
boos
a060666f6f