refactor(core): also make file_utils module private

Address review feedback from gemini-code-assist: file_utils is an
internal utility not used by external consumers, so it should be
private alongside parsers, rules, and schemas.

Author: avifenesh

crates/agnix-core/src/lib.rs

@@ -12,7 +12,7 @@
 
 pub mod config;
 pub mod diagnostics;
-pub mod file_utils;
+mod file_utils;
 pub mod fixes;
 mod parsers;
 mod rules;