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

.github/workflows/release.yml

@@ -24,13 +24,15 @@ jobs:
         uses: actions/setup-go@v6
         with:
           go-version-file: go.work
-          cache-dependency-path: backend/go.sum
+          cache-dependency-path: |
+            backend/go.sum
+            versionctl/go.sum
 
       - name: Verify tag and VERSION
         id: version
         run: |
-          version=$(go run ./backend/cmd/versionctl print)
-          go run ./backend/cmd/versionctl verify-tag "${GITHUB_REF_NAME}"
+          version=$(go run ./versionctl print)
+          go run ./versionctl verify-tag "${GITHUB_REF_NAME}"
           printf 'version=%s\n' "$version" >> "$GITHUB_OUTPUT"
 
   build-linux:
@@ -47,7 +49,9 @@ jobs:
         uses: actions/setup-go@v6
         with:
           go-version-file: go.work
-          cache-dependency-path: backend/go.sum
+          cache-dependency-path: |
+            backend/go.sum
+            versionctl/go.sum
 
       - name: Setup Bun
         uses: oven-sh/setup-bun@v2
@@ -93,7 +97,9 @@ jobs:
         uses: actions/setup-go@v6
         with:
           go-version-file: go.work
-          cache-dependency-path: backend/go.sum
+          cache-dependency-path: |
+            backend/go.sum
+            versionctl/go.sum
 
       - name: Setup Bun
         uses: oven-sh/setup-bun@v2
@@ -153,7 +159,9 @@ jobs:
         uses: actions/setup-go@v6
         with:
           go-version-file: go.work
-          cache-dependency-path: backend/go.sum
+          cache-dependency-path: |
+            backend/go.sum
+            versionctl/go.sum
 
       - name: Setup Bun
         uses: oven-sh/setup-bun@v2