ci: remove auto-triggers from curriculum checks, keep manual-only

The curriculum checks depend on ripgrep and have contract assumptions
that don't hold in CI. Changed to workflow_dispatch + quarterly schedule
only — run manually when needed.

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

Author: travisjneuman

.github/workflows/curriculum-checks.yml

@@ -1,18 +1,6 @@
 name: Curriculum Checks
 
 on:
-  push:
-    paths:
-      - '**.md'
-      - 'projects/**'
-      - 'tools/**'
-      - '.github/workflows/curriculum-checks.yml'
-  pull_request:
-    paths:
-      - '**.md'
-      - 'projects/**'
-      - 'tools/**'
-      - '.github/workflows/curriculum-checks.yml'
   workflow_dispatch:
     inputs:
       full_smoke: