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/__init__.py

@@ -0,0 +1,14 @@
+"""
+Built-in extractor registration.
+
+Importing this module registers all built-in extractors with the global registry.
+"""
+
+from markitect.proxy.registry import registry
+from markitect.proxy.extractors.pdf import PdfExtractor
+from markitect.proxy.extractors.html import HtmlExtractor
+from markitect.proxy.extractors.markdown import MarkdownNormalizer
+
+registry.register(PdfExtractor())
+registry.register(HtmlExtractor())
+registry.register(MarkdownNormalizer())