[FEATURE]: Make the agent event schema the default path #626

Summary

Once every producer emits the schema, make it the only path a run's events take. Producers currently emit it alongside the brokers it duplicates, so nothing yet depends on it.

The coupling this removes is visible in the standalone CLI: with no HTTP involved it still builds server state in process, calls the web-server handlers, and re-parses the resulting SSE bytes, because there is no way to consume an agent without serializing to SSE first. Making a run's events readable from its handle is what lets a consumer stop doing that.

Consumers reading the stream directly is also what makes the request-scoped brokers redundant, so retiring them and the identity they are keyed by belongs here rather than earlier.

The SSE handler keeps projecting onto its existing wire schema so behavior holds. Replacing that projection with a producer built on the event stream is #576, the A2A equivalent is #577, and the CLI moves off its in-process SSE round trip with them.

Goals

• A run's events reaching its observer directly, rather than through process-global brokers.
• The run's stream carrying response content as well, so an observer sees what an agent is saying and not only what it is doing. Clients read StreamItem for text and tool calls today, which is what keeps them on the old path.
• The per-request registries retired, since a run's handle is what holds its channels once consumers read from it.
• Run identity independent of HTTP, so a request id is no longer what a run is keyed by. The registries are what key on it, so they go together.
• Cancellation reaching MCP through the run's handle rather than a separate process-global registry.

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