Declare the streaming endpoints in the contract

[ExtraToast]

What to build

The four real-time endpoints this service exposes appear in its contract.

Three of them are HTTP and can be declared directly as event-stream responses: the session event feed, the workspace runner event feed, and the chat answer stream. The fourth is a WebSocket used for terminal attach, which OpenAPI cannot express — document it where the service's contract documentation lives, so it stops being invisible.

These endpoints are in production and consumed by the agents UI today, and nothing currently detects a change to their event shapes. Declaring them buys documentation and breaking-change coverage; it does not make them fuzzable.

Acceptance criteria

• The three HTTP streaming endpoints appear in the committed spec with their event content type
• The WebSocket endpoint is documented, including its URL shape and message contract
• The breaking-change check covers the streaming endpoints from now on
• Export drift passes with the new paths in place

Blocked by

• Adopt the customizer and annotate the domain error codes