feat(core): declare module structure

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

crates/core/src/io/file_reader.rs

@@ -0,0 +1,13 @@
+use crate::error::Result;
+use std::path::Path;
+
+pub struct FileReader {/* TODO */}
+
+impl FileReader {
+    pub fn open(_path: &Path) -> Result<Self> {
+        todo!()
+    }
+    pub fn file_size(&self) -> u64 {
+        todo!()
+    }
+}

crates/core/src/io/line_index.rs

@@ -0,0 +1 @@
+pub struct LineIndex {/* TODO */}

crates/core/src/io/mod.rs

@@ -0,0 +1,2 @@
+pub mod file_reader;
+pub mod line_index;