Audit AI

Sweep behaviour trees for references that no longer resolve. The flagship is a node pointing at a blackboard key that has been renamed or deleted: there is no compile error, no reference viewer entry and nothing red in the editor, and at runtime the decorator built on it evaluates as FALSE rather than failing, so the branch it guards silently never runs and the AI looks badly designed instead of broken. Also keys that resolve but are the wrong type for the node, trees with no root, and keys nothing appears to use.