Automatically register tasks at global level

When a user declares an `exec` task, go ahead and register the task at global level, to reduce boilerplate. Similar to how tasks work in `make`.

No files, tests, or entry points are named. Start by locating how an `exec` task is declared and how tasks are registered globally, then compare the behavior with make-style task registration; done means an `exec` declaration is globally available without the stated boilerplate and existing task behavior remains intact.

Written by the indexing model from the issue text.