feat(02-03): implement ZhihuAdapter for answer and zhuanlan content

- Implement ZhihuAdapter class inheriting PlatformAdapter
- Support answer URL content extraction
- Support zhuanlan (article) URL content extraction
- Add cookie-based authentication from KBSettings
- Detect login page redirects (expired cookies)
- Return AuthError for missing/expired cookies
- Format content with YAML frontmatter
- Update package exports to include ZhihuAdapter

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

kb_pipeline/__init__.py

@@ -25,6 +25,7 @@ from .config import KBSettings, get_settings
 from .retry import retry_on_network_error
 from .link_parser import LinkParser, ParsedLink
 from .idempotency import IdempotencyChecker
+from .platforms import PlatformAdapter, BilibiliAdapter, ZhihuAdapter
 
 __all__ = [
     # Schemas
@@ -54,6 +55,10 @@ __all__ = [
     "ParsedLink",
     # Idempotency
     "IdempotencyChecker",
+    # Platform Adapters
+    "PlatformAdapter",
+    "BilibiliAdapter",
+    "ZhihuAdapter",
 ]
 
 __version__ = "0.1.0"

kb_pipeline/platforms/__init__.py

@@ -12,8 +12,10 @@ Each adapter implements the PlatformAdapter interface:
 
 from .base import PlatformAdapter
 from .bilibili import BilibiliAdapter
+from .zhihu import ZhihuAdapter
 
 __all__ = [
     "PlatformAdapter",
     "BilibiliAdapter",
+    "ZhihuAdapter",
 ]