fix(#2822): properly cleanup tmux sessions on process shutdown

Two issues fixed:
1. process-cleanup.ts used fire-and-forget void Promise for shutdown
   handlers — now properly collects and awaits all cleanup promises
   via Promise.allSettled, with dedup guard to prevent double cleanup
2. TmuxSessionManager was never registered for process cleanup —
   now registered in create-managers.ts via registerManagerForCleanup

Also fixed setTimeout().unref() which could let the process exit
before cleanup completes.

src/create-managers.ts

@@ -7,6 +7,7 @@ import { BackgroundManager } from "./features/background-agent"
 import { SkillMcpManager } from "./features/skill-mcp-manager"
 import { initTaskToastManager } from "./features/task-toast-manager"
 import { TmuxSessionManager } from "./features/tmux-subagent"
+import { registerManagerForCleanup } from "./features/background-agent/process-cleanup"
 import { createConfigHandler } from "./plugin-handlers"
 import { log } from "./shared"
 import { markServerRunningInProcess } from "./shared/tmux/tmux-utils/server-health"
@@ -30,6 +31,14 @@ export function createManagers(args: {
   markServerRunningInProcess()
   const tmuxSessionManager = new TmuxSessionManager(ctx, tmuxConfig)
 
+  registerManagerForCleanup({
+    shutdown: async () => {
+      await tmuxSessionManager.cleanup().catch((error) => {
+        log("[create-managers] tmux cleanup error during process shutdown:", error)
+      })
+    },
+  })
+
   const backgroundManager = new BackgroundManager(
     ctx,
     pluginConfig.background_task,