Import docs: adoption playbook + superseded-mappers note + loader status

Follow-up to the mapper retirement (the new docs missed the prior commit's
staging). Adds docs/IMPORT-ADOPTION.md (two-layer import story + stage/crosswalk
guidance), scripts/migration/README.md (dir superseded, points at the API +
loader), and updates the WJ loader README to complete status.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/migration/README.md

@@ -0,0 +1,24 @@
+# scripts/migration (superseded)
+
+The direct-SQL migrators that used to live here (`migrate_assets.py`,
+`migrate_communications.py`, `migrate_notifications.py`, `migrate_usb.py`,
+`run_migration.py`, `verify_migration.py`) and `scripts/import_from_mysql.py`
+were **removed** - they had drifted badly out of sync with the current schema
+(they targeted a nonexistent `equipment` table, the retired `Machine` model, and
+columns that no longer exist) and were actively misleading.
+
+## Use the import API + a site loader instead
+
+Legacy data is imported through the maintained, schema-agnostic contract in
+[`docs/IMPORT-API.md`](../../docs/IMPORT-API.md) (create through real endpoints
+with an admin PAT + `X-Import-Mode`). Each adopting site writes a thin loader
+that reads its own source database and drives that API.
+
+- Reference implementation: [`scripts/site_imports/wjf/`](../site_imports/wjf/)
+  (West Jefferson classic-ASP -> flask).
+- Adoption playbook: [`docs/IMPORT-ADOPTION.md`](../../docs/IMPORT-ADOPTION.md).
+
+## What's kept here
+
+- `fix_legacy_schema.sql` - one-time SQL fixups against a legacy source DB.
+- `one-offs/` - individual one-shot SQL snippets (see its README).