fix(background-agent): clean tmux cancel bookkeeping

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

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

src/features/background-agent/manager.test.ts

@@ -15,6 +15,7 @@ afterAll(() => { mock.restore() })
 import { getSessionPromptParams, clearSessionPromptParams } from "../../shared/session-prompt-params-state"
 import { tmpdir } from "node:os"
 import type { PluginInput } from "@opencode-ai/plugin"
+import { _resetForTesting as resetClaudeCodeSessionState, subagentSessions } from "../claude-code-session-state"
 import type { BackgroundTask, ResumeInput } from "./types"
 import { MIN_IDLE_TIME_MS } from "./constants"
 import { BackgroundManager } from "./manager"
@@ -213,6 +214,10 @@ function getCompletionTimers(manager: BackgroundManager): Map<string, ReturnType
   return (manager as unknown as { completionTimers: Map<string, ReturnType<typeof setTimeout>> }).completionTimers
 }
 
+function getRootDescendantCounts(manager: BackgroundManager): Map<string, number> {
+  return (manager as unknown as { rootDescendantCounts: Map<string, number> }).rootDescendantCounts
+}
+
 function getQueuesByKey(
   manager: BackgroundManager
 ): Map<string, Array<{ task: BackgroundTask; input: import("./types").LaunchInput }>> {
@@ -2585,6 +2590,7 @@ describe("BackgroundManager - Non-blocking Queue Integration", () => {
 
     test("should keep task cancelled when cancelled during tmux callback before running state is assigned", async () => {
       // given
+      resetClaudeCodeSessionState()
       const originalTmuxEnvironment = process.env.TMUX
       process.env.TMUX = "test-session"
 
@@ -2674,7 +2680,10 @@ describe("BackgroundManager - Non-blocking Queue Integration", () => {
         expect(promptAsyncSessionIDs).not.toContain(createdSessionID)
         expect(abortCalls).toEqual([createdSessionID])
         expect(getConcurrencyManager(manager).getCount("test-agent")).toBe(0)
+        expect(getRootDescendantCounts(manager).has("parent-session")).toBe(false)
+        expect(subagentSessions.has(createdSessionID)).toBe(false)
       } finally {
+        resetClaudeCodeSessionState()
         if (originalTmuxEnvironment === undefined) {
           delete process.env.TMUX
         } else {

src/features/background-agent/manager.ts

@@ -493,6 +493,10 @@ export class BackgroundManager {
 
     if (this.tasks.get(task.id)?.status === "cancelled") {
       await this.abortSessionWithLogging(sessionID, "cancelled during tmux setup")
+      subagentSessions.delete(sessionID)
+      if (task.rootSessionID) {
+        this.unregisterRootDescendant(task.rootSessionID)
+      }
       this.concurrencyManager.release(concurrencyKey)
       return
     }