fix(background-agent): handle message.part.delta for heartbeat (OpenCode >=1.2.0)

OpenCode 1.2.0+ changed reasoning-delta and text-delta to emit
'message.part.delta' instead of 'message.part.updated'. Without
handling this event, lastUpdate was only refreshed at reasoning-start
and reasoning-end, leaving a gap where extended thinking (>3min)
could trigger stale timeout.

Accept both event types as heartbeat sources for forward compatibility.

src/features/background-agent/background-event-handler.ts

@@ -52,7 +52,7 @@ export function handleBackgroundEvent(args: {
 
   const props = event.properties
 
-  if (event.type === "message.part.updated") {
+  if (event.type === "message.part.updated" || event.type === "message.part.delta") {
     if (!props || !isRecord(props)) return
     const sessionID = getString(props, "sessionID")
     if (!sessionID) return