Use toml for config file

この issue には説明がありません。

The issue title suggests changing the configuration file format to TOML. First, locate the current config file handling in the codebase, likely in a config/ or internal/ directory. Identify the existing format (e.g., JSON, YAML) and understand how it's parsed and used. Research Go TOML libraries (e.g., BurntSushi/toml) and plan the migration, ensuring backward compatibility if needed. Check for any tests related to config loading to update.

索引モデルが issue の本文から書いたものです。