Setup Agent Discovery Signals

Create the global custom column behind each observable trigger.

Takes no body. A signal is defined ONCE for the account and every list patches the same column - that is what makes cross-campaign analysis possible at all, and why the column is global rather than list-scoped.

Triggers nothing can observe come back as research rather than being quietly created: they are real and useful, but they belong in the copy rather than in the targeting. Promising to watch something nothing watches is a program that never fires and a failure nobody can see.