Prepending a constant value to sublists of a list
Rewriting other's using operator forms
I like @eldo's solution, but it can be shorter!
Map[Prepend[-1], list, {2}]
Same for @aardvark2012's
Map[Insert[-1, 1], list, {2}]
Solutions of my own
list[[All, All, 0]] = Prepend[-1]@*List;
Or better
list[[All, All, 0]] = {-1, ##} &;
Also as an operator
ReplacePart[{_, _, 0} -> ({-1, ##} &)]
Best other
Given that the OP accepted mine (hastily to my taste and despite the good quality of the others), I want to highlight this answer given by Mr.Wizard in a comment to another answer. Easy to understand, and short:
Map@Prepend[-1] /@ list
☺ = {-1, ##} & @@@ # & /@ # &;
☺ @ list
(* thanks: Mr.Wizard *)
If we're golfing ;-)
Prepend[-1]/@#&/@list