Mike Burgess pointed me at the 2nd Council of Orange, whose canons may be read from a Reformed website here. Relevant canons with respect to our cooperation with grace include these:
CANON 18. That grace is not preceded by merit. Recompense is due to good works if they are performed; but grace, to which we have no claim, precedes them, to enable them to be done.I missed these when preparing my previous post, but the Catholic Encyclopedia points out that the 2nd Council of Orange intended to address "the current errors concerning the doctrine of grace and free will, i.e. Semipelagianism" and that the Council's work was heavily influenced by the writings of St. Augustine. See also a separate, somewhat lengthy article on semi-Pelagianism in the Encyclopedia. I don't have time to review these two sources in their entirety right now, but I will later, and I'll either update this post with more information from them or add a new post if it seems worthwhile to do so.
CANON 20. That a man can do no good without God. God does much that is good in a man that the man does not do; but a man does nothing good for which God is not responsible, so as to let him do it (emphasis added).
Lastly, in a trio of comments in the combox to this article at Dave Armstrong's blog, Ben M. has provided a number of relevant and helpful additional quotations from St. Augustine related to our cooperation with grace. You may find them here, here, and here. Thanks to Mike and Ben for the information, and thanks again to Dave for the "advertising" :-)