✨ feat(vscode): 摘修复识别 alembic 迁移，接不上发布分支链的高亮禁用不可摘

按 writing-alembic-migrations 的规矩判断：迁移的前任必须已在发布分支上，
且发布分支上该前任不能已有别的后继；根迁移接不上已有链、删迁移会断链也不可摘。
列表里带迁移的提交打「迁移」角标，不可摘的高亮警告色、禁用勾选并写明原因；
确认接口再校验一遍，绕过前端也摘不进去。

Claude-Session: https://claude.ai/code/session_01QWxLb6X8Vd2G5vHnp6R7qm

vscode/package.json

@@ -2,7 +2,7 @@
   "publisher": "clurdra",
   "name": "superpowers-vscode-clurdra",
   "displayName": "Superpowers-clurdra",
-  "version": "0.2.121",
+  "version": "0.2.123",
   "packageManager": "pnpm@10.27.0",
   "description": "Superpowers specs and plans Kanban explorer",
   "author": "clurdra",