fix(plugin): normalize event session ids

Handle OpenCode session events that carry the session ID under properties.info.id or properties.info.sessionID so background tasks and continuation hooks do not miss idle/error/delete events.

Add regression coverage for nested session.idle events completing background tasks and waking continuation hooks.

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

@@ -1,12 +1,8 @@
 import { log } from "../../shared"
+import { resolveSessionEventID } from "../../shared/event-session-id"
 import { MIN_IDLE_TIME_MS } from "./constants"
 import type { BackgroundTask } from "./types"
 
-function getString(obj: Record<string, unknown>, key: string): string | undefined {
-  const value = obj[key]
-  return typeof value === "string" ? value : undefined
-}
-
 export function handleSessionIdleBackgroundEvent(args: {
   properties: Record<string, unknown>
   findBySession: (sessionID: string) => BackgroundTask | undefined
@@ -26,7 +22,7 @@ export function handleSessionIdleBackgroundEvent(args: {
     emitIdleEvent,
   } = args
 
-  const sessionID = getString(properties, "sessionID")
+  const sessionID = resolveSessionEventID(properties)
   if (!sessionID) return
 
   const task = findBySession(sessionID)