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>

.gitignore

@@ -3,9 +3,5 @@
 /resources/_gen/
 /.hugo_build.lock
 
-# Migration scratch space: originals pulled off the old WordPress host.
-# Only the files that get copied into content/, assets/ and static/ are committed.
+# Local scratch space
 /.cache/
-
-__pycache__/
-*.py[cod]