Add the agent_session_lease table with one row for each agent session.
A turn will take the lease in the transaction that records its execution,
so that only one turn runs on a session at a time.
The row keeps its epoch after release and after session deletion, so the
epoch of a thread ID only increases.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
