feat: add engineering conventions and CI gates documentation

- Introduced a new document outlining SDK version locking, static analysis, formatting, generated artifacts management, branching and commit conventions, and CI gate checks.
- Updated README to include the new conventions document.
- Modified API design to use numeric error codes instead of strings, with a dedicated ErrorCode object for better maintainability.
- Adjusted global exception handling to return numeric error codes.
- Updated tests to reflect changes in error code handling.

backend/10-testing.md

@@ -101,7 +101,7 @@ class StoreControllerTest {
             content = """{}"""
         }.andExpect {
             status { isBadRequest() }
-            jsonPath("$.code") { value("INVALID_PARAM") } // 对应 06-api-design.md 的 ApiResult 结构
+            jsonPath("$.code") { value(ErrorCode.INVALID_PARAM) } // 对应 06-api-design.md 的 ApiResult 结构
         }
     }
 }