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/html.py

@@ -0,0 +1,45 @@
+"""
+HTML extractor using markdownify.
+"""
+
+from pathlib import Path
+
+from markitect.proxy.extractors.base import BaseExtractor
+from markitect.proxy.models import ExtractionResult
+from markitect.proxy.exceptions import DependencyMissingError
+
+
+class HtmlExtractor(BaseExtractor):
+    """Converts HTML files to Markdown via markdownify."""
+
+    name = "html"
+    version = "1.0"
+    extensions = (".html", ".htm")
+
+    def check_dependencies(self) -> bool:
+        try:
+            import markdownify  # noqa: F401
+            return True
+        except ImportError:
+            return False
+
+    def dependency_hint(self) -> str:
+        return 'pip install "markitect[proxy-html]"  (or: pip install markdownify)'
+
+    def extract(self, source_path: Path) -> ExtractionResult:
+        if not self.check_dependencies():
+            raise DependencyMissingError(
+                "markdownify is required to extract HTML files.",
+                package="markdownify",
+                install_hint=self.dependency_hint(),
+            )
+
+        import markdownify
+
+        html_content = source_path.read_text(encoding="utf-8")
+        md_text = markdownify.markdownify(html_content, heading_style="ATX")
+        return ExtractionResult(
+            content=md_text,
+            extractor=self.name,
+            extractor_version=self.version,
+        )