Update AGENTS.md's def-order error note when bendlang/bend#1783 ships #6

A call to a def written below its caller reports "an unfilled law is a dead claim" (bendlang/bend#1782, filed 2026-10-20). The fix, bendlang/bend#1783, opened 2026-10-20, makes the error name the file order. It has no review yet.

AGENTS.md teaches the old wording: '"an unfilled law is a dead claim" is a call above its def'. When the minimum Bend includes the fix, replace that note with the new message.