Fix broken hash tests to work with current CLI architecture:
- Update import to use @import(src) module system
- Add hash module export to utils.zig
- Make validatePath() public for testing
- Fix Zig 0.15 API: writeFile options struct, var tmp_dir for cleanup
- Fix file paths: use tmp_dir realpath for hashFile
- Replace std.fs.MAX_PATH_BYTES with 4096 buffer
All hash tests now passing.