fix(tokenstore): abort a stuck keyring read on Ctrl-C

The OS keyring call ran on context.Background(), so a Ctrl-C during a slow/blocked read (Keychain subprocess, or a headless box with no keyring daemon) left the user waiting out the full 5s timeout — or hanging.

Listen for SIGINT for the duration of the call and return early wrapping context.Canceled (which flows into the CLI's silent user-abort exit). signal.Notify fans out to every channel, so the process's own handler still cancels the root context. A per-request context can't be threaded here: the store is reached via auth-go's Store interface, which carries no context.Context.