feat: Add configurable ProcessMode (detect-only / analyze / full)

Introduces a ProcessMode enum that controls how far the PDF pipeline
runs, enabling fast document triage without paying extraction or
markdown conversion costs. Exposed via --detect-only and --analyze
flags in both pdf2md and detect-pdf CLIs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/integration_tests.rs

@@ -267,6 +267,7 @@ fn test_markdown_options_custom() {
         include_images: false,
         include_links: false,
         include_page_numbers: false,
+        ..Default::default()
     };
     assert!(!opts.detect_headers);
     assert!(opts.detect_lists);