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>

pyproject.toml

@@ -32,6 +32,9 @@ capabilities = [
 development = [
     "kaizen-agentic @ file:./capabilities/kaizen-agentic"
 ]
+proxy-pdf = ["pymupdf4llm>=0.0.10"]
+proxy-html = ["markdownify>=0.13.1"]
+proxy = ["pymupdf4llm>=0.0.10", "markdownify>=0.13.1"]
 
 [project.scripts]
 markitect = "markitect.cli:main"