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).

.gitignore

@@ -0,0 +1,25 @@
+# ── Spring / Maven 타겟 ──────────────────────────────────────────────
+target/
+*.class
+*.jar
+!.mvn/wrapper/maven-wrapper.jar
+
+# ── 레거시 C 빌드 산출물 ─────────────────────────────────────────────
+build/
+bin/
+*.o
+*.a
+*.so
+core
+
+# ECPG 생성 소스 (.pgc -> .c)
+legacy/app/dbio/*.c
+legacy/app/online/mg_recv_svc.c
+legacy/app/online/ac_intake_svc.c
+legacy/app/batch/*.c
+
+# ── 기타 ─────────────────────────────────────────────────────────────
+*.log
+.idea/
+.vscode/
+*.iml