- Add scopedBus.clear() in dispose() to prevent subscription accumulation
on worker crash/restart cycles
- Use two-arg subscribe() overload when filter is null instead of passing
empty object; fix filter type to include null
- Update ASCII flow diagram: onEvent is a notification, not request/response
Plugin workers register event handlers via `ctx.events.on()` in the SDK,
but these subscriptions were never forwarded to the host process. The host
sends events via `notifyWorker("onEvent", ...)` which produces a JSON-RPC
notification (no `id`), but the worker only dispatched `onEvent` as a
request handler — notifications were silently dropped.
Changes:
- Add `events.subscribe` RPC method so workers can register subscriptions
on the host-side event bus during setup
- Handle `onEvent` notifications in the worker notification dispatcher
(previously only `agents.sessions.event` was handled)
- Add `events.subscribe` to HostServices interface, capability map, and
host client handler
- Add `subscribe` handler in host services that registers on the scoped
plugin event bus and forwards matched events to the worker
