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/registry.py

@@ -0,0 +1,65 @@
+"""
+Extractor registry — register and look up extractors by file extension.
+"""
+
+from __future__ import annotations
+
+import logging
+from typing import Dict, List, TYPE_CHECKING
+
+from pathlib import Path
+
+from markitect.proxy.exceptions import ExtractorNotFoundError
+
+if TYPE_CHECKING:
+    from markitect.proxy.extractors.base import BaseExtractor
+
+logger = logging.getLogger("markitect.proxy.registry")
+
+
+class ExtractorRegistry:
+    """Maps file extensions to their corresponding extractors."""
+
+    def __init__(self):
+        self._extractors: Dict[str, BaseExtractor] = {}
+
+    def register(self, extractor: BaseExtractor) -> None:
+        """Register an extractor for all of its declared extensions."""
+        for ext in extractor.extensions:
+            ext_lower = ext.lower()
+            self._extractors[ext_lower] = extractor
+            logger.debug("Registered %s extractor for %s", extractor.name, ext_lower)
+
+    def get_extractor(self, extension: str) -> BaseExtractor:
+        """Look up an extractor by file extension (e.g. ``'.pdf'``).
+
+        Raises:
+            ExtractorNotFoundError: If no extractor handles the extension.
+        """
+        ext_lower = extension.lower()
+        if ext_lower not in self._extractors:
+            known = ", ".join(sorted(self._extractors.keys()))
+            raise ExtractorNotFoundError(
+                f"No extractor registered for {ext_lower!r}. "
+                f"Supported extensions: {known}",
+                context={"extension": ext_lower},
+            )
+        return self._extractors[ext_lower]
+
+    def get_extractor_for_file(self, path: Path) -> BaseExtractor:
+        """Look up an extractor for a file based on its suffix."""
+        return self.get_extractor(path.suffix)
+
+    def list_extractors(self) -> List[BaseExtractor]:
+        """Return a de-duplicated list of registered extractors."""
+        seen = set()
+        result = []
+        for ext in self._extractors.values():
+            if id(ext) not in seen:
+                seen.add(id(ext))
+                result.append(ext)
+        return result
+
+
+# Module-level singleton
+registry = ExtractorRegistry()