Flattens brain module structure

Renames the `mod.rs` file for the `brain` module to `brain.rs`.
This simplifies the module path and aligns with standard Rust practices for modules defined entirely within a single file.

src/rustylee/src/brain.rs

@@ -1,4 +1,5 @@
 pub mod state;
+
 pub use self::state::State;
 use tracing::info;
 use std::thread::JoinHandle;