in the keystone.js, in keystone.init(), remove the 'bbcode' and include 'code'. This will resolve the issue. I am using the following configurations for a client, you can use it if you like, I have added all the essential settings to TinyMCE:
keystone.init({
'name': '',
'brand': '',
'sass': 'public',
'static': 'public',
'favicon': 'public/favicon.ico',
'views': 'templates/views',
'view engine': 'hbs',
'custom engine': handlebars.create({
layoutsDir: 'templates/views/layouts',
partialsDir: 'templates/views/partials',
defaultLayout: 'default',
helpers: new require('./templates/views/helpers')(),
extname: '.hbs',
}).engine,
'emails': 'templates/emails',
'auto update': true,
'session': true,
'auth': true,
'user model': 'User',
'wysiwyg override toolbar': false,
'wysiwyg menubar': true,
'wysiwyg skin': 'lightgray',
'wysiwyg additional buttons': 'searchreplace visualchars, code'
+ ' charmap ltr rtl pagebreak paste, forecolor backcolor,'
+' emoticons media, preview, image',
'wysiwyg additional plugins': 'table, advlist, anchor,'
+ ' autolink, autosave, code, charmap, contextmenu, '
+ ' directionality, emoticons, fullpage, hr, media, pagebreak,'
+ ' paste, preview, searchreplace, textcolor,'
+ ' visualblocks, visualchars, wordcount, image',
});