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/plugin-honcho/src/lib.rs

@@ -892,7 +892,7 @@ impl core_api::plugin::Plugin for HonchoPlugin {
         self.honcho_memory.activate(Arc::clone(&client), workspace_id.clone(), Arc::clone(&user_config));
 
         let session_map  = Arc::clone(&self.honcho_memory.session_map);
-        let mut rx       = ctx.event_bus.subscribe();
+        let mut rx       = ctx.chat_bus.subscribe();
         let cancel       = CancellationToken::new();
         let cancel_clone = cancel.clone();
         let running      = Arc::clone(&self.running);