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>

hugo.toml

@@ -18,8 +18,8 @@ buildFuture = false
   dateFormat = '2. January 2006'
 
 [markup.goldmark.renderer]
-  # The imported WordPress content contains hand-written HTML (tables, links with
-  # target attributes) that must survive rendering.
+  # Some pages contain hand-written HTML (tables, links with target attributes)
+  # that must survive rendering.
   unsafe = true
 
 [markup.tableOfContents]