feat(15-01): add enable_semantic_segmentation field to KBSettings

- Add enable_semantic_segmentation: bool field with default=True
- Allow override via KB_ENABLE_SEMANTIC_SEGMENTATION env var
- Pydantic automatically parses boolean strings (true/false, yes/no, 1/0)
- Update docstring to document new field

CONFIG-01: LLM segmentation can be enabled/disabled via configuration

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

kb_workflow/pipeline/config.py

@@ -62,6 +62,10 @@ class KBSettings(BaseSettings):
     overwrite_archives: bool = Field(
         default=False, description="覆盖已存在的归档文件（非交互模式）"
     )
+    enable_semantic_segmentation: bool = Field(
+        default=True,
+        description="启用 LLM 语义分段（默认启用）"
+    )
 
 
 @lru_cache