Add .env file support for easier configuration

[RC-CHN]

Motivation / 动机

Using command-line arguments to pass API keys and model settings can be inconvenient during testing and daily use. This PR adds support for reading configuration from .env files, making it easier to manage settings without repeatedly setting environment variables.

在测试和日常使用中，通过命令行参数传入 API 密钥和模型设置略有不便。本 PR 添加了从 .env 文件读取配置的功能，无需重复设置环境变量即可轻松管理配置。

Changes / 变更内容

• Add python-dotenv dependency in pyproject.toml
• Modify nanocoder/config.py to auto-load .env files (with graceful fallback if dotenv not installed)
• Add .env.example with bilingual comments (Chinese/English)
• Update README.md and README_CN.md with .env usage instructions

Usage / 使用方法

cp .env.example .env
# Edit .env with your API key and model settings
nanocoder