Add a session store
Implement SessionStore without changing Runtime. AgentMuru 0.2 requires:
create,get,list, andsavefor session records;append_messagefor explicit ordered message writes;create_run,update_run, andget_runfor durable transitions;get_idempotent_runandbind_idempotency_keyfor session-scoped replay;recover_interrupted_runsfor honest process-restart semantics;append_event,events, and compatible replay/follow subscription behavior.
append_event must allocate the next sequence atomically. Persist, commit, then publish.
Never assign a sequence from an in-memory object when multiple store clients can write.
Use tests/sessions/test_store_contract.py as an adapter contract. A production adapter
also needs ownership queries, retention, encryption, backups, migrations, safe errors, and
reconnect subscriptions. See the 0.2 custom-store migration.