feat: add config loading module with YAML support

实现应用配置加载模块，支持从 YAML 文件或默认值加载配置。
覆盖 ChromaDB、嵌入模型、文档分块和 HTTP 服务配置。

- src/core/config.py: 配置数据类 (AppConfig/ChromaConfig/EmbedConfig/ChunkConfig/ServerConfig) + load_config 函数
- tests/test_config.py: 5 个单元测试覆盖默认值、API 模式和文件加载

Co-Authored-By: Claude <noreply@anthropic.com>

src/core/config.py

@@ -0,0 +1,68 @@
+"""应用配置加载模块."""
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from pathlib import Path
+
+import yaml
+
+
+@dataclass
+class ChromaConfig:
+    """ChromaDB 配置."""
+
+    persist_dir: str = "./data"
+    collection_name: str = "markdown_docs"
+
+
+@dataclass
+class EmbedConfig:
+    """嵌入模型配置."""
+
+    mode: str = "local"  # "local" | "api"
+    local_model: str = "BAAI/bge-small-zh-v1.5"
+    api_base: str = ""
+    api_key: str = ""
+
+
+@dataclass
+class ChunkConfig:
+    """文档分块配置."""
+
+    max_size: int = 1000
+    overlap: int = 100
+
+
+@dataclass
+class ServerConfig:
+    """HTTP 服务配置."""
+
+    host: str = "0.0.0.0"
+    port: int = 8000
+
+
+@dataclass
+class AppConfig:
+    """应用总配置."""
+
+    chroma: ChromaConfig = field(default_factory=ChromaConfig)
+    embed: EmbedConfig = field(default_factory=EmbedConfig)
+    chunk: ChunkConfig = field(default_factory=ChunkConfig)
+    server: ServerConfig = field(default_factory=ServerConfig)
+
+    def __init__(self, **kwargs):
+        self.chroma = ChromaConfig(**kwargs.get("chroma", {}))
+        self.embed = EmbedConfig(**kwargs.get("embed", {}))
+        self.chunk = ChunkConfig(**kwargs.get("chunk", {}))
+        self.server = ServerConfig(**kwargs.get("server", {}))
+
+
+def load_config(path: str | None = None) -> AppConfig:
+    """从 YAML 文件加载配置, 若文件不存在则返回默认配置."""
+    config_path = path or "config.yaml"
+    if not Path(config_path).exists():
+        return AppConfig()
+
+    with open(config_path, "r", encoding="utf-8") as f:
+        data = yaml.safe_load(f) or {}
+    return AppConfig(**data)