chore: Lock down docs deployment to main branch only

- Remove migrate-tutorials branch from deployment triggers
- Remove PR builds entirely
- Only build and deploy when tutorials/ directory changes on main
- Manual workflow dispatch still available for emergency deployments

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.github/workflows/deploy-docs.yml

@@ -2,11 +2,6 @@ name: Deploy MyST Documentation
 
 on:
   push:
-    branches: [main, migrate-tutorials]
-    paths:
-      - "tutorials/**"
-      - ".github/workflows/deploy-docs.yml"
-  pull_request:
     branches: [main]
     paths:
       - "tutorials/**"
@@ -57,8 +52,8 @@ jobs:
           path: "tutorials/_build/html"
 
   deploy:
-    # Only deploy from main and migrate-tutorials branches, not from PRs
-    if: github.event_name == 'push' && (github.ref == 'refs/heads/main' || github.ref == 'refs/heads/migrate-tutorials')
+    # Only deploy from main branch
+    if: github.event_name == 'push' && github.ref == 'refs/heads/main'
     environment:
       name: github-pages
       url: ${{ steps.deployment.outputs.page_url }}