✨ feat(vscode): 工单移交的会话包改走阿里云 OSS、换 xz 压缩，两边进度一步步显示

- 会话包传到 OSS 的 spx-handoff/<owner>/<repo>/issue-<N>-<时间>.tar.xz，上传时对象设成私有；不再挂 Gitea 工单附件
- 新命令「配置移交存储」：endpoint、bucket 存扩展设置，AccessKey 存密钥存储；没配时移交 / 接管直接提示，什么都不改
- 共享 state 字段 handoffAttachmentId 改名 handoffBundleKey（含 spx CLI 与 schema）
- 打包换成 xz 1 级（tar -cJf），去掉 100MB 上限检查和先试压一遍的重复压缩
- 发送方提示打包 / 上传了多少 MB；接管方同一条提示依次显示下载、解包、建 worktree、装会话，完成前不消失
- 会话交接（单会话）仍走 Gitea 附件，Gitea 附件白名单没有 .xz，单独保留 gzip 打包

vscode/webview-ui/src/components/IssueDetailPanel.tsx

@@ -918,7 +918,7 @@ export function IssueDetailPanel({
             avoid acting on a gitea issue of the same number. */}
         {issue.source !== 'youtrack' && !readOnly && (
           <>
-            {issue.handoffAttachmentId && me !== undefined && (issue.assignees ?? []).includes(me) && (
+            {issue.handoffBundleKey && me !== undefined && (issue.assignees ?? []).includes(me) && (
               <button
                 type="button"
                 data-tooltip={`接管 ${issue.handoffFrom ?? '?'} 的移交`}
@@ -930,7 +930,7 @@ export function IssueDetailPanel({
                 {isHandoffRunning(issue.number) ? <Loader2 className="size-3.5 animate-spin" /> : <Download className="size-3.5" />}
               </button>
             )}
-            {!issue.handoffAttachmentId && issue.column !== 'done' && ((issue.assignees ?? []).length === 0 || (me !== undefined && (issue.assignees ?? []).includes(me))) && (
+            {!issue.handoffBundleKey && issue.column !== 'done' && ((issue.assignees ?? []).length === 0 || (me !== undefined && (issue.assignees ?? []).includes(me))) && (
               <button
                 type="button"
                 data-tooltip="移交给同事"
@@ -942,7 +942,7 @@ export function IssueDetailPanel({
                 {isHandoffRunning(issue.number) ? <Loader2 className="size-3.5 animate-spin" /> : <ArrowRightLeft className="size-3.5" />}
               </button>
             )}
-            {issue.handoffAttachmentId && !(me !== undefined && (issue.assignees ?? []).includes(me)) && (
+            {issue.handoffBundleKey && !(me !== undefined && (issue.assignees ?? []).includes(me)) && (
               <span
                 className="inline-flex shrink-0 items-center justify-center rounded border border-[var(--vscode-panel-border)] p-1 text-xs opacity-70"
                 data-tooltip={`${issue.handoffFrom ?? '?'} 已移交，等 ${(issue.assignees ?? []).join(', ') || '?'} 接管`}