ssh-pageant now has its first release, version 1.0 — fulfill your authentication agent needs between Cygwin’s OpenSSH and Pageant today! It’s documented, and pre-built binaries are available. And there will be cake, of course.
Any “1.0” feels like it must have some magical aura, but in the end it is just a number. The program works as expected, and has the features I felt were needed to be complete, so 1.0 it is. The next step is to get this packaged and included in the official Cygwin repositories, which I would welcome volunteer help in doing.