fix(ci): revert to MIGRATION_PAT for variable migration

GITHUB_TOKEN lacks permission to manage repository variables.
Matches the working SpokeToWork approach — requires MIGRATION_PAT secret.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Dev User

.github/workflows/migrate-secrets-to-vars.yml

@@ -4,16 +4,13 @@ name: Migrate Secrets to Variables (one-time)
 on:
   workflow_dispatch:
 
-permissions:
-  actions: write
-
 jobs:
   migrate:
     runs-on: ubuntu-latest
     steps:
       - name: Migrate secrets to variables
         env:
-          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GH_TOKEN: ${{ secrets.MIGRATION_PAT }}
           REPO: ${{ github.repository }}
           # --- Non-sensitive values to migrate ---
           S_NEXT_PUBLIC_AUTHOR_AVATAR: ${{ secrets.NEXT_PUBLIC_AUTHOR_AVATAR }}