feat(06-02): move pipeline core files to kb_workflow.pipeline

- Move pipeline.py, schemas.py, config.py, state.py, errors.py
- Move retry.py, idempotency.py, link_parser.py
- Create pipeline/__init__.py with updated imports
- Update all internal imports to use kb_workflow.pipeline prefix

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

kb_workflow/pipeline/idempotency.py

@@ -0,0 +1,80 @@
+"""Idempotency Checker module
+
+Provides file existence checking to avoid reprocessing content.
+Checks for metadata JSON and content Markdown files.
+"""
+
+from pathlib import Path
+from typing import Tuple, List
+
+
+class IdempotencyChecker:
+    """Checks if content has already been processed
+
+    Determines if the output files for a given content already exist,
+    enabling idempotent processing to avoid duplicate work.
+
+    File naming convention:
+        - Metadata: {platform}_{content_id}_metadata.json
+        - Content: {platform}_{content_id}_content.md
+    """
+
+    def check_exists(
+        self,
+        output_dir: Path,
+        content_id: str,
+        platform: str
+    ) -> Tuple[bool, List[Path]]:
+        """Check if output files already exist for this content
+
+        Args:
+            output_dir: Directory where output files are stored
+            content_id: Unique content identifier (e.g., BV1GJ411x7Qw)
+            platform: Platform name (e.g., bilibili, zhihu)
+
+        Returns:
+            Tuple of (all_exist, existing_files):
+                - all_exist: True if both metadata and content files exist
+                - existing_files: List of Path objects for existing files
+        """
+        existing_files: List[Path] = []
+
+        if not content_id or not platform:
+            return (False, existing_files)
+
+        # Define expected file paths
+        metadata_file = output_dir / f"{platform}_{content_id}_metadata.json"
+        content_file = output_dir / f"{platform}_{content_id}_content.md"
+
+        # Check if files exist
+        if metadata_file.exists():
+            existing_files.append(metadata_file)
+
+        if content_file.exists():
+            existing_files.append(content_file)
+
+        # Return True only if both files exist
+        all_exist = len(existing_files) == 2
+
+        return (all_exist, existing_files)
+
+    def get_expected_files(
+        self,
+        output_dir: Path,
+        content_id: str,
+        platform: str
+    ) -> Tuple[Path, Path]:
+        """Get the expected file paths for a content
+
+        Args:
+            output_dir: Directory where output files are stored
+            content_id: Unique content identifier
+            platform: Platform name
+
+        Returns:
+            Tuple of (metadata_path, content_path)
+        """
+        metadata_path = output_dir / f"{platform}_{content_id}_metadata.json"
+        content_path = output_dir / f"{platform}_{content_id}_content.md"
+
+        return (metadata_path, content_path)