The honcho memory sink needs to observe every user's completed chat turns
from one subscription, keyed by ChatEvent.user_id. But each UserContext
minted its own per-user ChatEventBus, so a single global subscription saw
nothing.
- UserContext now publishes onto the shared Runtime.event_bus (the one
Skald::subscribe_chat_events reads) instead of a fresh per-user bus.
- Expose that bus to plugins as PluginContext.chat_bus (distinct from
system_bus, which carries only infra lifecycle events).
- plugin-honcho subscribes via ctx.chat_bus.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>