Execute a flow or agent

Main dispatch endpoint for flow and agent execution with record resolution. Supports streaming via SSE or synchronous JSON responses. Send the RFC 7240 `Prefer: respond-async` header to receive a durable execution handle immediately, then poll its status URL for completion. A saved agent may run on the durable session lane; the admission headers `x-runtype-concurrency` (reject, supersede, queue, join), `x-runtype-coalesce` (true) and `idempotency-key` then apply exactly as on `POST /v1/agents/{id}/execute`, and are ignored for in-process execution except join, which fails explicitly on unsupported lanes. Join accepts only new user-message deltas, preserves the active execution configuration and authority, and returns a deliveryId with a deliveryStatusUrl. Prefer: respond-async with join returns a native delivery receipt without queuing an HTTP replay.