Ghost 0.7.2 added a first ‘beta’ version of API access. This gives access to the new {{get}} helper and the embeddable ghost.url.api() helper for making ajax requests.
These must be enabled via a checkbox on the labs page in your Ghost admin panel.
Editing partials/navigation.hbs:
<ul>
{{#get "tags" limit="all"}}
{{#foreach tags}}
<li><a href="{{url}}">{{name}}</a></li>
{{/foreach}}
{{/get}}
</ul>