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>

layouts/_partials/func/featured-image.html

@@ -1,7 +1,7 @@
 {{- /*
   Resolve a page's featured image to a Resource.
-  Looks at the `featured` front-matter key first (set by the WordPress import),
-  then falls back to the first image in the page bundle.
+  Looks at the `featured` front-matter key first, then falls back to the first
+  image in the page bundle.
   Returns the Resource, or nothing.
 */ -}}
 {{- $img := "" -}}