Isn't calling Jesus "God" a Greek idea, foreign to the Hebrew Bible?

This is a fair and serious challenge, and frankly the church earned it: for centuries we explained Yeshua's deity almost entirely through Nicaea and Chalcedon, homoousios and "two natures," vocabulary Abraham would not have recognized. So you are right that the packaging is Greek. And you are right to guard Israel's confession that God is one; that instinct is not an obstacle to the truth, it is the floor it stands on.

But the deity itself is not a Greek import, and here is where it actually lives. The Tanakh already knows a figure who will not stay a mere agent: the Malach YHWH, the Angel of YHWH, who speaks in the first person as God ("you have not withheld your son from Me," Genesis 22:12), who carries the Name itself ("My Name is within him," Exodus 23:21), and who receives the reverence owed to God alone. The synagogue called Him the Memra, the Word of YHWH. And Daniel sees a Son of Man come on the clouds, a divine prerogative, and receive the worship of all nations (Daniel 7:13-14). When John writes that "the Word became flesh" (John 1:14), he is not importing Athens. He is saying the One Israel already knew took on a body. The councils reached for Greek words because that was the air they breathed; the reality was Hebrew the whole time.

Do not take it from me. Read Genesis 22:11-12, Exodus 23:20-21, and Daniel 7:13-14, and ask whether that figure is a creature or the LORD Himself.