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/hooks/stop-continuation-guard/hook.ts

@@ -5,6 +5,7 @@ import {
   clearContinuationMarker,
   setContinuationMarkerSource,
 } from "../../features/run-continuation-state"
+import { resolveSessionEventID } from "../../shared/event-session-id"
 import { log } from "../../shared/logger"
 
 const HOOK_NAME = "stop-continuation-guard"
@@ -86,11 +87,11 @@ export function createStopContinuationGuardHook(
     const props = event.properties as Record<string, unknown> | undefined
 
     if (event.type === "session.deleted") {
-      const sessionInfo = props?.info as { id?: string } | undefined
-      if (sessionInfo?.id) {
-        clear(sessionInfo.id)
-        clearContinuationMarker(ctx.directory, sessionInfo.id)
-        log(`[${HOOK_NAME}] Session deleted: cleaned up`, { sessionID: sessionInfo.id })
+      const sessionID = resolveSessionEventID(props)
+      if (sessionID) {
+        clear(sessionID)
+        clearContinuationMarker(ctx.directory, sessionID)
+        log(`[${HOOK_NAME}] Session deleted: cleaned up`, { sessionID })
       }
     }
   }