Under File, select "Prototypes", then double-click "Wigger", then select "Script..." from the Prototype menu.

The majority of the drawing-related logic is in the "wiggler" Prototype; choose Tool -> Widgets from the menu, double-click on the main drawing surface, click "Prototype..." and then choose "Prototype -> Script..." from the menu.

Various other buttons and interactive elements contain their own scripts; from the main view in "Widgets" editing mode you can double-click on the ball-point pen tool and then click "Script..." to see how tool-changing works, for example.

Do you want to keep tools internal sort of like the uxn/varvara ecosystem's focus on dogfooding tal, or are external tools ok?

If it's possible to build things for the Decker ecosystem using Decker or Lil, it's advantageous; it will be easier to keep things up to date and portable as the platform evolves, and to reuse bits and pieces for other Decker-related projects.

That said, anyone interested in building tooling is welcome to do so. This is why, for example, the Decker file format is fully documented: https://beyondloom.com/decker/format.html