The official way to put the hook at logging in / logging out is described in these Apple documents Customizing Login and Logout and Tech Note 2228. But I'm afraid that the log-out hook was no longer directly supported.
So, the second best way is to run a headless Cocoa app, and receive a notification from the system using NSWorkspace, see the document and the list of available notifications.
To make a headless Cocoa app, you need to set an entry in its Info.plist called LSUIElement, see here. By setting LSUIElement to be yes, the app don't appear in the dock, but perform various operation in the background.