Mac OS X: "A keychain cannot be found to store [credentials]"

Question

I'm trying to use git with a remote repo over the command line in OS X 10.8.4. When I enter my credentials for GitHub every time I perform a git push -u origin master, I get a system saying "A keychain cannot be found to store [my GitHub credentials]." I'm given the option to reset my keychain or continue without storing the creds.

I tried following the instructions given on Apple's site for this situation – although they're a bit outdated.

When I try to reset my keychain, I'm prompted for my login password, which is systematically rejected by the authentication dialog.

I've never had this kind of issue; what's the cause, and how can it be fixed?

Shown in the Keychain Access app:

• Login

• System

• System roots

In the ~/LIbrary/Preferences file:

login.keychain
metadata.keychain
two hidden files with seemingly random alphanumeric names

The permissions for ~/Library/Preferences are

-rw-r--r--

For my personal account, and no permissions are shown for root upon ls -l. This may be the root of the problem; I was trying to push using root.