Move Canvas conversation stream transport into the TypeScript SDK

Desired Behavior

Canvas still implements the Agent Server conversation WebSocket transport, including authentication, reconnects, and handshake timeouts. That transport belongs in the SDK TypeScript client so local and Docker conversations use the same protocol implementation.

Status: ready-for-dev

Acceptance Criteria

• Export a browser-compatible conversation event stream client from the secondary TypeScript clients entrypoint.
• Own first-frame authentication (no key in the URL), replay query parameters, bounded reconnect backoff, handshake timeout, send, and cleanup in the SDK.
• Preserve Canvas lifecycle callbacks and prevent replaced sockets or cancelled reconnect timers from changing the active connection.
• Cover authentication ordering, reconnection/cancellation, stale events, and timeout behavior with focused tests; verify against the isolated Agent Server.
• Document the API and migrate Canvas to consume it in a separate PR.