After learning much about code signing and reviewing the problems and solutions in the Apple Dev forums I determined the "Apple Development" signing identity Xcode generated automatically failed to properly install in my Login Keychain so I manually generated a new one which is working.

Thanks for the quick response. The underlying problem is getting Xcode to build my app successfully. This was working until I unwittingly damaged my Login Keychain and had to rebuild it. The basic test above isolates the problem to code signing in general as opposed to some setting in Xcode. What changed is I had to reset my Login Keychain so some piece of information may have been lost but neither codesign nor "automatically manage signing" in Xcode provide any clue in this case.