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/compaction-todo-preserver/hook.ts

@@ -1,4 +1,5 @@
 import type { PluginInput } from "@opencode-ai/plugin"
+import { resolveSessionEventID } from "../../shared/event-session-id"
 import { log } from "../../shared/logger"
 
 interface TodoSnapshot {
@@ -97,8 +98,7 @@ async function resolveTodoWriter(): Promise<TodoWriter | null> {
 }
 
 function resolveSessionID(props?: Record<string, unknown>): string | undefined {
-  return (props?.sessionID ??
-    (props?.info as { id?: string } | undefined)?.id) as string | undefined
+  return resolveSessionEventID(props)
 }
 
 export interface CompactionTodoPreserver {