fix(background-agent): prevent zombie processes by aborting sessions on shutdown (#1240) (#1243)

- BackgroundManager.shutdown() now aborts all running child sessions via
  client.session.abort() before clearing state, preventing orphaned
  opencode processes when parent exits
- Add onShutdown callback to BackgroundManager constructor, used to
  trigger TmuxSessionManager.cleanup() on process exit signals
- Interactive bash session hook now aborts tracked subagent opencode
  sessions when killing tmux sessions (defense-in-depth)
- Add 4 tests verifying shutdown abort behavior and callback invocation

Closes #1240

src/index.ts

@@ -264,6 +264,11 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
       });
       log("[index] onSubagentSessionCreated callback completed");
     },
+    onShutdown: () => {
+      tmuxSessionManager.cleanup().catch((error) => {
+        log("[index] tmux cleanup error during shutdown:", error)
+      })
+    },
   });
 
   const atlasHook = isHookEnabled("atlas")