GET / (WebSocket upgrade) overview

WebSocket upgrade endpoint. Authenticated via initial Authorization header or via a follow-up `auth` frame within unauthenticatedTimeoutMs. After auth the server emits a `hello` frame and the connection accepts protocol-v1 client frames (subscribe/unsubscribe/session.subscribe/session.unsubscribe/session.start/session.message/ping/hook.decision/pairing.register/pairing.consume) and emits server frames (hello/error/pong/run.event/hook.request/hook.resolved/pairing.consumed). Source: packages/agent-mux/gateway/src/server.ts:570-680, 733-825.