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/markdown.py

@@ -0,0 +1,29 @@
+"""
+Markdown normalizer — passes through Markdown with minimal transformation.
+
+No external dependencies required.
+"""
+
+from pathlib import Path
+
+from markitect.proxy.extractors.base import BaseExtractor
+from markitect.proxy.models import ExtractionResult
+
+
+class MarkdownNormalizer(BaseExtractor):
+    """Normalizes other Markdown files (built-in, no optional deps)."""
+
+    name = "markdown"
+    version = "1.0"
+    extensions = (".md", ".markdown", ".mdown")
+
+    def check_dependencies(self) -> bool:
+        return True
+
+    def extract(self, source_path: Path) -> ExtractionResult:
+        content = source_path.read_text(encoding="utf-8")
+        return ExtractionResult(
+            content=content,
+            extractor=self.name,
+            extractor_version=self.version,
+        )