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/task-reminder/hook.ts

@@ -1,5 +1,7 @@
 import type { PluginInput } from "@opencode-ai/plugin"
 
+import { resolveSessionEventID } from "../../shared/event-session-id"
+
 const TASK_TOOLS = new Set([
   "task",
   "task_create",
@@ -50,8 +52,7 @@ export function createTaskReminderHook(_ctx: PluginInput) {
     "tool.execute.after": toolExecuteAfter,
     event: async ({ event }: { event: { type: string; properties?: unknown } }) => {
       if (event.type !== "session.deleted") return
-      const props = event.properties as { info?: { id?: string } } | undefined
-      const sessionId = props?.info?.id
+      const sessionId = resolveSessionEventID(event.properties)
       if (!sessionId) return
       sessionCounters.delete(sessionId)
     },