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/cli.py

@@ -7147,6 +7147,13 @@ try:
 except ImportError:
     pass  # Helper module not available
 
+# Register proxy file system commands
+try:
+    from markitect.proxy.cli import proxy_group
+    cli.add_command(proxy_group)
+except ImportError:
+    pass  # Proxy module not available
+
 # Make cli function available as main entry point
 main = cli