refactor: decouple InvokableAgent from A2A dependency

[pgrayy]

Motivation

Consumers of the multiagent module (Graph, Swarm) were transitively pulling in @a2a-js/sdk types because InvokableAgent.stream() yielded AgentStreamEvent | A2AStreamEvent. This made the A2A package effectively required for anyone using multi-agent orchestration, even when no A2A agents were involved.

Changes

InvokableAgent.stream() now yields StreamEvent, an empty abstract base class with no fields. This removes the transitive dependency on A2A types from the public interface.

To preserve type-safe event handling in multi-agent orchestration, NodeStreamUpdateEvent.event is renamed to NodeStreamUpdateEvent.inner with a new NodeInnerEvent discriminated union type. The union is tagged by source:

type NodeInnerEvent =
  | (AgentStreamEvent & { readonly source: 'agent' })
  | (Exclude<MultiAgentStreamEvent, NodeStreamUpdateEvent> & { readonly source: 'multiAgent' })
  | (StreamEvent & { readonly source: 'custom' })

Consumers switch on inner.source first, then inner.type for full discriminated union narrowing:

for await (const event of graph.stream('hello')) {
  if (event.type === 'nodeStreamUpdateEvent') {
    if (event.inner.source === 'agent') {
      // event.inner is AgentStreamEvent, narrows on .type
      if (event.inner.type === 'modelStreamUpdateEvent') { ... }
    }
  }
}

A2AExecutor uses instanceof narrowing internally since it receives StreamEvent from InvokableAgent.stream(). This is an internal implementation detail, not consumer-facing.

Breaking Changes

• NodeStreamUpdateEvent.event renamed to NodeStreamUpdateEvent.inner
• NodeStreamUpdateEvent.inner type changed from AgentStreamEvent | A2AStreamEvent | ... to NodeInnerEvent (source-tagged union)
• InvokableAgent.stream() yield type changed from AgentStreamEvent to StreamEvent