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, steer), 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 steer, which fails explicitly on unsupported lanes. x-runtype-concurrency: auto sends the same user-message deltas as steer and lets a classifier choose: while a turn runs, a message that belongs to it steers and an independent one queues as the next turn; with nothing running it is an ordinary turn with no route report. A routed or downgraded answer carries X-Runtype-Busy-Message-Route (steer or queue) and X-Runtype-Busy-Message-Source (classifier, shadow, abstained, timeout, error, unanswered, circuit_open, or disabled), plus X-Runtype-Busy-Message-Probability and X-Runtype-Busy-Message-Model when the classifier answered; JSON bodies repeat them as busyMessageRoute. Where auto cannot route (in-process execution, stored history, a flow, no conversationId, coalesce, Prefer: respond-async) it keeps the default admission and reports source disabled. Any other x-runtype-concurrency value is refused with 400 INVALID_CONCURRENCY_MODE. Steer accepts only new user-message deltas, preserves the active execution configuration and authority, and returns a deliveryId with a deliveryStatusUrl. Prefer: respond-async with steer returns a native delivery receipt without queuing an HTTP replay.