If you link an entire armature object from another .blend file, it's not possible to enter pose mode.
If you create a new armature object, then link to an armature data block from another file, you lose bone constraints.
You can link the whole object, the Duplicate Linked (Alt+D), and it will keep the original bone constraints. But later, if you change the constraints in the other .blend file, the changes won't propagate to the duplicate.
So, how do you safely animate a linked armature that has bone contraints? Do people just not do this? Do you instead make a copy of the armature in every .blend file, and keep changes in sync manually?