support standalone (.shr) activity download via form version

Branch download by the card form 'version' field: standalone packs a flat .shr (no xbot_robot/xbot_extensions, no internal deps), private keeps the current .bot. Also enable kaniko layer cache to skip pip install on code-only builds.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.woodpecker.yml

@@ -16,6 +16,8 @@ steps:
       tags: latest
       repo: woni/office-xbot-feishu
       registry: registry.woni.link
+      cache: true
+      cache_repo: woni/office-xbot-feishu-cache
       mirrors:
         - 5c492fmo.mirror.aliyuncs.com
       dockerfile: ./Dockerfile