chore: Phase 3a - CI pipeline to PR-only mode

- Removed 'push' trigger from ci.yml
- Now runs validators only on pull_requests
- Main branch validation handled by merge-to-main.yml
- Reduces duplicate validation runs on push

Next:
- Phase 3b: Group validators for parallel execution
- Phase 4: Secret management validation
- Phase 5: Monitoring & metrics

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -1,17 +1,13 @@
-name: Quant Engine CI/CD Pipeline
+name: Validators (Pull Requests Only)
 
 on:
-  push:
-    branches: [ main ]
   pull_request:
     branches: [ main ]
   workflow_dispatch:
 
-# ─────────────────────────────────────────────────────────────────
-# CI 역할: 코드 구조 검증 게이트 (순수 Python, yaml/json)
-#   - Validate Specs / Formula Registry / Coverage / Behavioral Coverage
-# 통합 테스트(run_release_dag, ingest 등)는 로컬 또는 클라우드 서버에서 실행
-# ─────────────────────────────────────────────────────────────────
+# Phase 3: Validator pipeline
+# Main branch push validation moved to merge-to-main.yml
+# This workflow runs validators only on PRs for feedback
 
 jobs:
   validate-core: