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

CHANGELOG.md

@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 - Narrowed agnix-core public API surface (#85)
-  - Made `parsers`, `rules`, and `schemas` modules private
+  - Made `parsers`, `rules`, `schemas`, and `file_utils` modules private
   - Re-exported `Validator` trait for custom validator implementations
   - No breaking changes for agnix-cli or external consumers using documented API
 

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;