chore(mpc-system): 添加 .gitignore 排除敏感配置

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

backend/mpc-system/.gitignore

@@ -0,0 +1,35 @@
+# Environment files (contain secrets)
+.env
+.env.local
+.env.production
+
+# Build artifacts
+/bin/
+*.exe
+*.dll
+*.so
+*.dylib
+
+# Test binary
+*.test
+
+# Output of go coverage
+*.out
+
+# IDE
+.idea/
+.vscode/
+*.swp
+*.swo
+
+# OS files
+.DS_Store
+Thumbs.db
+
+# Logs
+*.log
+logs/
+
+# Temporary files
+tmp/
+temp/