feat(06-02): move processors and platforms to kb_workflow.pipeline

- Move all processor implementations (parse, download, asr, analyze, archive)
- Move all platform adapters (base, bilibili, zhihu)
- Update all internal imports to use kb_workflow.pipeline prefix

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

kb_workflow/pipeline/platforms/__init__.py

@@ -0,0 +1,21 @@
+"""Platform Adapters for KB Pipeline
+
+This module provides platform-specific adapters for content extraction
+from various platforms (Bilibili, Zhihu, etc.).
+
+Each adapter implements the PlatformAdapter interface:
+- get_content_type: Identify content type from URL
+- fetch_metadata: Fetch content metadata
+- fetch_content: Fetch content body in Markdown format
+- get_output_filename: Generate output filename
+"""
+
+from kb_workflow.pipeline.platforms.base import PlatformAdapter
+from kb_workflow.pipeline.platforms.bilibili import BilibiliAdapter
+from kb_workflow.pipeline.platforms.zhihu import ZhihuAdapter
+
+__all__ = [
+    "PlatformAdapter",
+    "BilibiliAdapter",
+    "ZhihuAdapter",
+]