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/features/background-agent/process-cleanup.ts

@@ -11,7 +11,7 @@ function registerProcessSignal(
     handler()
     if (exitAfter) {
       process.exitCode = 0
-      setTimeout(() => process.exit(), 6000).unref()
+      setTimeout(() => process.exit(), 6000)
     }
   }
   process.on(signal, listener)
@@ -32,16 +32,26 @@ export function registerManagerForCleanup(manager: CleanupTarget): void {
   if (cleanupRegistered) return
   cleanupRegistered = true
 
+  let cleanupPromise: Promise<void> | undefined
+
   const cleanupAll = () => {
+    if (cleanupPromise) return
+    const promises: Promise<void>[] = []
     for (const m of cleanupManagers) {
       try {
-        void Promise.resolve(m.shutdown()).catch((error) => {
-          log("[background-agent] Error during async shutdown cleanup:", error)
-        })
+        promises.push(
+          Promise.resolve(m.shutdown()).catch((error) => {
+            log("[background-agent] Error during async shutdown cleanup:", error)
+          })
+        )
       } catch (error) {
         log("[background-agent] Error during shutdown cleanup:", error)
       }
     }
+    cleanupPromise = Promise.allSettled(promises).then(() => {})
+    cleanupPromise.then(() => {
+      log("[background-agent] All shutdown cleanup completed")
+    })
   }
 
   const registerSignal = (signal: ProcessCleanupEvent, exitAfter: boolean): void => {