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/exceptions.py

@@ -0,0 +1,40 @@
+"""
+Proxy-specific exceptions.
+
+Extends the MarkitectError hierarchy for proxy file operations.
+"""
+
+from typing import Optional, Dict, Any
+
+from markitect.exceptions import MarkitectError
+
+
+class ProxyError(MarkitectError):
+    """Base exception for all proxy operations."""
+    pass
+
+
+class ExtractorNotFoundError(ProxyError):
+    """No extractor registered for the given file extension."""
+    pass
+
+
+class DependencyMissingError(ProxyError):
+    """An extractor's optional dependency is not installed.
+
+    Attributes:
+        package: The missing Python package name.
+        install_hint: Suggested pip install command.
+    """
+
+    def __init__(
+        self,
+        message: str,
+        package: str = "",
+        install_hint: str = "",
+        cause: Optional[Exception] = None,
+        context: Optional[Dict[str, Any]] = None,
+    ):
+        super().__init__(message, cause=cause, context=context)
+        self.package = package
+        self.install_hint = install_hint