fix(todo-continuation-enforcer): defer prune interval to first idle event

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/hooks/todo-continuation-enforcer/dispose.test.ts

@@ -8,6 +8,7 @@ declare module "bun:test" {
 
 import { afterAll, afterEach, describe, expect, it, mock } from "bun:test"
 
+import type { BackgroundManager } from "../../features/background-agent"
 import * as actualSessionStateModule from "./session-state"
 import type { SessionStateStore } from "./session-state"
 
@@ -37,6 +38,12 @@ function createMockPluginInput(): PluginInput {
   } as PluginInput
 }
 
+function createMockBackgroundManager(): BackgroundManager {
+  return {
+    getTasksByParentSession: () => [{ status: "running" }],
+  } as BackgroundManager
+}
+
 function getCreatedSessionStateStore(): SessionStateStore {
   if (!createdSessionStateStore) {
     throw new Error("expected session state store to be created")
@@ -68,7 +75,7 @@ describe("todo-continuation-enforcer dispose", () => {
     enforcer.dispose()
   })
 
-  it("#given enforcer with active session states #when dispose is called #then internal session state store is shut down", () => {
+  it("#given enforcer with active session states #when dispose is called #then internal session state store is shut down", async () => {
     // given
     const originalClearInterval = globalThis.clearInterval
     const clearIntervalCalls: Array<Parameters<typeof clearInterval>[0]> = []
@@ -78,9 +85,13 @@ describe("todo-continuation-enforcer dispose", () => {
     }) as typeof clearInterval
 
     try {
-      const enforcer = createTodoContinuationEnforcer(createMockPluginInput())
+      const enforcer = createTodoContinuationEnforcer(createMockPluginInput(), {
+        backgroundManager: createMockBackgroundManager(),
+      })
       const sessionStateStore = getCreatedSessionStateStore()
 
+      await enforcer.handler({ event: { type: "session.idle", properties: { sessionID: "session-1" } } })
+
       enforcer.markRecovering("session-1")
       enforcer.markRecovering("session-2")