chore: acquire-core migration monorepo (legacy C + Spring Boot target skeleton)

TxCore/ECPG legacy 매입·정산 C 슬라이스(빌드검증) + Spring Boot 멀티모듈 골격(mvn test green) + MIGRATION.md 전환룰 + CI(boot 빌드).

.forgejo/workflows/ci.yml

@@ -0,0 +1,9 @@
+name: ci
+on: [push, pull_request]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: |
+          git clone "${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}.git" repo && cd repo && git checkout "$GITHUB_SHA"
+          cd boot && mvn -q -B -DskipTests=false test