feat(config): implement configuration management for issue #7

- Create config.py with Config class
- Config precedence: ENV > config file > defaults
- data_dir: JIGAIDO_DATA_DIR env or ~/.jigaido/config.json or default
- bot_token: JIGAIDO_BOT_TOKEN env var
- ensure_data_dir() method to create data directory
- Add tests/test_config.py with 7 passing tests

Fixes #7

config.py

@@ -0,0 +1,37 @@
+"""JIGAIDO Configuration Management."""
+
+import json
+import os
+from pathlib import Path
+from typing import Optional
+
+DEFAULT_DATA_DIR = Path.home() / ".jigaido"
+
+
+class Config:
+    """JIGAIDO configuration with precedence: ENV > config file > defaults."""
+
+    def __init__(self):
+        self.data_dir: Path = self._resolve_data_dir()
+        self.bot_token: Optional[str] = os.environ.get("JIGAIDO_BOT_TOKEN")
+
+    def _resolve_data_dir(self) -> Path:
+        env_dir = os.environ.get("JIGAIDO_DATA_DIR")
+        if env_dir:
+            return Path(env_dir)
+
+        config_file = Path("~/.jigaido/config.json").expanduser()
+        if config_file.exists():
+            with open(config_file) as f:
+                config_data = json.load(f)
+                if "data_dir" in config_data:
+                    return Path(config_data["data_dir"])
+
+        return DEFAULT_DATA_DIR
+
+    def ensure_data_dir(self) -> None:
+        """Ensure the data directory exists."""
+        self.data_dir.mkdir(parents=True, exist_ok=True)
+
+
+config = Config()