feat(proxy): add proxy file system for non-markdown source conversion

Introduces a new `markitect/proxy/` module with pluggable extractors that
convert non-markdown sources (PDF, HTML) into tracked markdown proxy files.
Proxy files preserve origin metadata (path, checksum, timestamp) so they
can be kept in sync when the original changes.

CLI commands: `proxy create`, `proxy update`, `proxy status`, `proxy extractors`.
Built-in extractors: PDF (pymupdf4llm), HTML (markdownify), Markdown (built-in).

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

markitect/proxy/extractors/base.py

@@ -0,0 +1,43 @@
+"""
+Abstract base class for proxy file extractors.
+"""
+
+from abc import ABC, abstractmethod
+from pathlib import Path
+
+from markitect.proxy.models import ExtractionResult
+
+
+class BaseExtractor(ABC):
+    """Base class that all proxy extractors must implement."""
+
+    name: str = ""
+    version: str = "1.0"
+    extensions: tuple = ()
+
+    @abstractmethod
+    def extract(self, source_path: Path) -> ExtractionResult:
+        """Extract markdown content from a source file.
+
+        Args:
+            source_path: Path to the source file.
+
+        Returns:
+            ExtractionResult with the extracted markdown content.
+        """
+
+    @abstractmethod
+    def check_dependencies(self) -> bool:
+        """Check whether all required dependencies are available.
+
+        Returns:
+            True if all dependencies are installed, False otherwise.
+        """
+
+    def dependency_hint(self) -> str:
+        """Human-readable install instructions for missing dependencies.
+
+        Returns:
+            A string like ``pip install markitect[proxy-pdf]``.
+        """
+        return ""