refactor(branch-dropdown): drop worktree parent-branch merge shortcut

Remove the parent-branch button shown beside the branch selector along
with its supporting API, command, service, model, and i18n strings.
Add migration m20260423 to drop the now-unused folder.parent_branch
column.

src/i18n/messages/en.json

@@ -1082,7 +1082,6 @@
       "noLocalBranches": "No local branches",
       "remoteBranches": "Remote branches ({count, plural, one {#} other {#}})",
       "noRemoteBranches": "No remote branches",
-      "parentBranchHint": "Current branch was created from {parentBranch}. Click to merge {parentBranch} into current branch.",
       "dialogs": {
         "newBranchTitle": "New branch",
         "newBranchDescription": "Create a new branch from current branch {branch}",