feat: 迁移 versionctl 为独立模块并新增 make version-bump 命令

- 将 backend/cmd/versionctl 和 backend/pkg/projectversion 迁移至独立 versionctl/ Go 模块
- 新增 bump 子命令支持 major/minor/patch 和指定版本号，含版本倒退防护
- 新增 make version-bump 编排完整升迁流程（bump + sync + check + commit + tag）
- 更新所有引用路径：根 Makefile、backend/Makefile、release.yml、.golangci.yml
- 新增 versionctl/.golangci.yml（精简配置）和 Makefile（lint/test/coverage）
- 根 Makefile lint/test 集成 versionctl 模块
- 同步 openspec specs：新增 version-bump spec，更新 release-pipeline spec

versionctl/.golangci.yml

@@ -0,0 +1,52 @@
+run:
+  timeout: 5m
+  tests: true
+
+linters:
+  disable-all: true
+  enable:
+    - errorlint
+    - errcheck
+    - staticcheck
+    - revive
+    - gocritic
+    - gosec
+    - nilerr
+    - goimports
+    - gocyclo
+
+linters-settings:
+  errcheck:
+    check-blank: true
+    check-type-assertions: true
+  revive:
+    rules:
+      - name: exported
+      - name: var-naming
+      - name: indent-error-flow
+      - name: error-strings
+      - name: error-return
+      - name: blank-imports
+  goimports:
+    local-prefixes: nex/versionctl
+  gocyclo:
+    min-complexity: 10
+
+issues:
+  exclude-generated: true
+  exclude-rules:
+    - path: '_test\.go'
+      linters:
+        - errcheck
+      source: '(^\s*_\s*=|,\s*_)'
+    - path: '_test\.go'
+      linters:
+        - revive
+      text: '^exported:'
+    - path: '_test\.go'
+      linters:
+        - gosec
+      text: 'G(101|401|501)'
+    - path: 'main\.go'
+      linters:
+        - gocyclo