When removing a keychain unlocker, if the keychain item doesn't exist
(e.g., already manually deleted or vault synced from another machine),
the removal should still succeed since the goal is to remove the
unlocker and the keychain item being gone already satisfies that goal.
- Replaced exec.Command calls to /usr/bin/security with native keybase/go-keychain API
- Added comprehensive test suite for keychain operations
- Fixed binary data storage in tests using hex encoding
- Updated macse tests to skip with explanation about ADE requirements
- All tests passing with CGO_ENABLED=1