fix(todo-continuation-enforcer): allow background task sessions to receive todo-continuation

Background task sessions registered in subagentSessions were not receiving
todo-continuation prompts, causing a deadlock: background tasks waited for
continuation that never came.

Changes:
- Allow both main session and background task sessions to receive continuation
- Add test for background task session continuation behavior
- Cleanup subagentSessions in test setup/teardown

This fixes the deadlock introduced in commit 116a90d which added todo waiting
logic to background-agent/manager.ts.

🤖 Generated with assistance of OhMyOpenCode

src/hooks/todo-continuation-enforcer.ts

@@ -1,7 +1,7 @@
 import { existsSync, readdirSync } from "node:fs"
 import { join } from "node:path"
 import type { PluginInput } from "@opencode-ai/plugin"
-import { getMainSessionID } from "../features/claude-code-session-state"
+import { getMainSessionID, subagentSessions } from "../features/claude-code-session-state"
 import {
   findNearestMessageWithFields,
   MESSAGE_STORAGE,
@@ -265,8 +265,11 @@ export function createTodoContinuationEnforcer(
       log(`[${HOOK_NAME}] session.idle`, { sessionID })
 
       const mainSessionID = getMainSessionID()
-      if (mainSessionID && sessionID !== mainSessionID) {
-        log(`[${HOOK_NAME}] Skipped: not main session`, { sessionID })
+      const isMainSession = sessionID === mainSessionID
+      const isBackgroundTaskSession = subagentSessions.has(sessionID)
+      
+      if (mainSessionID && !isMainSession && !isBackgroundTaskSession) {
+        log(`[${HOOK_NAME}] Skipped: not main or background task session`, { sessionID })
         return
       }