。
She gave me a signal not to say anything.
Literal
She [topic-は] anything [also-も] saying [contrast-は] must-not [quotative-と] signal [object-を] gave-me.
合図をくれた fronts くれる as a main verb — 'gave me a signal' — rather than the more familiar auxiliary use (~てくれる). The choice of くれる over the neutral 与える makes the action feel like a small kindness directed at the speaker, fitting the conspiratorial tone. Inside the quotative と clause, ~てはいけない conveys prohibition; together it's an embedded order delivered silently. Notice how Japanese reports a non-verbal communication act using exactly the same と construction it uses for spoken speech.