chore: Simplify codeowners rules (#1178)

Author: TaylorMutch

.github/CODEOWNERS

@@ -1,9 +1,5 @@
 # Broad ownership — core team reviews everything
 * @NVIDIA/openshell-codeowners @mrunalp @maxamillion @derekwaynecarr
 
-# Agent infrastructure — tighter review
-.agents/ @NVIDIA/openshell-codeowners
-AGENTS.md @NVIDIA/openshell-codeowners
-
 # Vouch list — maintainers only (bot commits bypass, but manual edits need review)
 .github/VOUCHED.td @NVIDIA/openshell-codeowners