Honcho: route per-user chat turns onto shared bus, expose to plugins

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>

crates/skald-core/src/plugin/mod.rs

@@ -187,6 +187,7 @@ impl PluginManager {
             api_provider_registry:   Arc::clone(skald.provider_registry()) as _,
             location:                Arc::clone(skald.location_manager()) as _,
             system_bus:              Arc::clone(skald.system_bus()),
+            chat_bus:                Arc::clone(skald.event_bus()),
             user_channel:            self.skald()? as Arc<dyn core_api::user_channel::UserChannelApi>,
             user_config:             Arc::clone(&self.user_config) as _,
             i18n:                    self.i18n(),