Remove migration tooling and documentation

The import is verified and content/ is now the source of truth, so the
one-shot WordPress importer under tools/ and MIGRATION.md are no longer
needed. Also drops the remaining references to the old site from the
README, config comments and data-file headers.

Build is unchanged: 38 pages, 718 internal links, none broken.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

assets/css/main.css

@@ -3,7 +3,7 @@
    cannot compile Sass. */
 
 :root {
-  --ink: #404040;              /* carried over from the old HitMag custom CSS */
+  --ink: #404040;
   --ink-soft: #6b6b6b;
   --ink-faint: #8c8c8c;
   --accent: #9c1c1c;