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/tmux-subagent/session-created-handler.ts

@@ -2,6 +2,7 @@ import type { PluginInput } from "@opencode-ai/plugin"
 import type { TmuxConfig } from "../../config/schema"
 import type { CapacityConfig, TrackedSession } from "./types"
 import { log } from "../../shared"
+import { resolveSessionEventID } from "../../shared/event-session-id"
 import { queryWindowState } from "./pane-state-querier"
 import { decideSpawnActions, type SessionMapping } from "./decision-engine"
 import { executeActions } from "./action-executor"
@@ -44,9 +45,9 @@ export async function handleSessionCreated(
   if (event.type !== "session.created") return
 
   const info = event.properties?.info
-  if (!info?.id || !info?.parentID) return
+  const sessionId = resolveSessionEventID(event.properties)
+  if (!sessionId || !info?.parentID) return
 
-  const sessionId = info.id
   const title = info.title ?? "Subagent"
 
   if (deps.sessions.has(sessionId) || deps.pendingSessions.has(sessionId)) {