Stream account agent events (SSE)

Server-Sent Events stream of agent build changes for an account. Requires membership of the account. Opens with a `ready` event, then sends an `agent.build` event whenever a build changes state, and a `heartbeat` every 25 seconds. An `agent.build` event names the agent that changed in `agent`, with `build_id` and `status` when they apply. Treat it as a signal to refetch the agent's build status: the API stays the authority, and an event the client misses costs a late refresh rather than a wrong screen. Each `agent.build` event carries an `id`. Reconnect with that value in the `Last-Event-ID` header, or in the `last_event_id` query parameter when the client cannot set headers, to receive the events it missed. Replay is capped; when it is, `ready` carries `"resync": true`, meaning the client is not guaranteed current and should refetch. Events for an agent an account deploys from arrive on that account's stream as well as the publisher's.