feat(sitemap): add XML sitemap generation module

Implement src/sitemap.rs with generate_sitemap() for SEO-compliant
XML sitemap generation. Follows the feed.rs pattern:

- SitemapEntry struct for URL metadata
- build_sitemap_xml() for XML construction
- xml_escape() for special character handling
- 5 unit tests covering single/multiple entries, lastmod, escaping

Module declared in main.rs but not yet integrated into pipeline.

flake.nix

@@ -46,6 +46,7 @@
               pkgs.taplo
               pkgs.pkg-config
               pkgs.nixfmt
+              pkgs.nodePackages.prettier
               pkgs.miniserve # Dev server for testing
             ]
             ++ pkgs.lib.optionals pkgs.stdenv.isDarwin [