docs: mark sixth-form flag pipeline change implemented

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

docs/superpowers/specs/2026-07-07-exam-phase-taxonomy-design.md

@@ -144,7 +144,7 @@ and is the only source that correctly classifies:
 - schools whose statutory age range extends to 18 on paper but which have no
   registered post-16 provision.
 
-### Pipeline change (future work)
+### Pipeline change (implemented 2026-07-07)
 
 1. `stg_gias_establishments.sql`: add
    `"OfficialSixthForm (name)" as official_sixth_form`.
@@ -152,6 +152,9 @@ and is the only source that correctly classifies:
    `has_sixth_form boolean` = `official_sixth_form = 'Has a sixth form'`.
 3. Expose `has_sixth_form` on the school API payloads.
 
+Implemented in `feat/gias-sixth-form-flag` — see
+`docs/superpowers/plans/2026-07-07-gias-sixth-form-flag.md`.
+
 ### Current heuristic — audit of `age_range` ~ "18" sites
 
 All must migrate to the `has_sixth_form` flag once exposed: