Session Primitive

Add the Session primitive — a durable, task-bound, bidirectional I/O channel that outlives a single run and acts as the run manager for chat.agent. Ships the Postgres Session + SessionRun tables, ClickHouse sessions_v1 + replication service, the sessions JWT scope, and the public CRUD + realtime routes (/api/v1/sessions, /realtime/v1/sessions/:session/:io) including end-and-continue for server-orchestrated run handoffs and session-stream waitpoints.