As I understand it, to share keychain data between a Watch app and the iOS app, you need to set up a keychain access group entitlement, and make the keychain item with the kSecAttrAccessGroup attribute and a value that matches that keychain access group entititlement.
Xcode sets up an entitlement file that looks like "$(AppIdentifierPrefix)some.identifier.here". In code, though, how do we get an app identifier prefix to make the matching kSecAttrAccessGroup value? Or does the kSecAttrAccessGroup value exclude the $(AppIdentifierPrefix) part of the entitlement item?