feat(pipeline): ingest GIAS OfficialSixthForm into staging

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

pipeline/plugins/extractors/tap-uk-gias/tap_uk_gias/tap.py

@@ -33,6 +33,7 @@ class GIASEstablishmentsStream(Stream):
         th.Property("EstablishmentName", th.StringType),
         th.Property("TypeOfEstablishment (name)", th.StringType),
         th.Property("PhaseOfEducation (name)", th.StringType),
+        th.Property("OfficialSixthForm (name)", th.StringType),
         th.Property("LA (code)", th.StringType),
         th.Property("LA (name)", th.StringType),
         th.Property("EstablishmentNumber", th.StringType),