feat: add .env file support for easier configuration

Load .env from project root (or parent dirs up to ~) on startup.
Existing env vars are not overridden. python-dotenv is now a
regular dependency.

Closes #1

Author: he-yufeng

README.md

@@ -54,7 +54,7 @@ Every pattern is a real, runnable implementation — not a diagram or a blog pos
 pip install nanocoderagent
 ```
 
-Pick your model — any OpenAI-compatible API works:
+Pick your model — any OpenAI-compatible API works. You can `export` env vars or drop a `.env` file in your project root:
 
 ```bash
 # Kimi K2.5

README_CN.md

@@ -54,7 +54,7 @@ Claude Code 51 万行源码提炼出来的 7 个核心模式：
 pip install nanocoderagent
 ```
 
-选你的模型，任何 OpenAI 兼容 API 都行：
+选你的模型，任何 OpenAI 兼容 API 都行。可以 `export` 环境变量，也可以在项目根目录放一个 `.env` 文件：
 
 ```bash
 # Kimi K2.5

nanocoder/config.py

@@ -2,6 +2,27 @@
 
 import os
 from dataclasses import dataclass
+from pathlib import Path
+
+
+def _load_dotenv():
+    """Load .env from cwd, walking up to home dir. No-op if python-dotenv missing."""
+    try:
+        from dotenv import load_dotenv
+        # search cwd first, then parent dirs up to ~
+        env_path = Path(".env")
+        if not env_path.exists():
+            cur = Path.cwd()
+            home = Path.home()
+            while cur != home and cur != cur.parent:
+                candidate = cur / ".env"
+                if candidate.exists():
+                    env_path = candidate
+                    break
+                cur = cur.parent
+        load_dotenv(env_path, override=False)
+    except ImportError:
+        pass  # python-dotenv not installed, silently skip
 
 
 @dataclass
@@ -15,6 +36,8 @@ class Config:
 
     @classmethod
     def from_env(cls) -> "Config":
+        # load .env if present (won't override existing env vars)
+        _load_dotenv()
         # pick up common env vars automatically
         api_key = (
             os.getenv("NANOCODER_API_KEY")

pyproject.toml

@@ -25,6 +25,7 @@ dependencies = [
     "openai>=1.0",
     "rich>=13.0",
     "prompt_toolkit>=3.0",
+    "python-dotenv>=1.0",
 ]
 
 [tool.hatch.build.targets.wheel]