fix(worktrees): detect external merges, clean up empty dirs, prune in owning repo

- ContentInMain: HEAD ancestor of the default branch or squash-merged
  (merge-tree == target tree), against local origin/<default> without fetch;
  list_worktrees mergedIntoMain and the overview modal ("merged (external)")
  use it, "Cleanup finished" takes clean Active rows that are in main
- cleanup_task_worktree skips the dirty check for a non-git directory
- prune / worktree remove / branch -D run in the repo owning the worktree
  (git-common-dir), not the list's working dir; branch already gone reports
  branchDeleted=true

src/ClaudeDo.Localization/locales/de.json

@@ -555,6 +555,8 @@
       "resolve": "Lösen",
       "phantom": "Phantom",
       "phantomTooltip": "Verzeichnis fehlt auf der Festplatte",
+      "mergedExternal": "gemergt (extern)",
+      "mergedExternalTooltip": "Die Commits sind schon im Default-Branch (lokaler origin-Stand, ohne Fetch), z.B. per Squash-PR. \"Abgeschlossene aufräumen\" entfernt ihn.",
       "ctxShowDiff": "Diff anzeigen",
       "ctxOpenInExplorer": "Im Explorer öffnen",
       "ctxJumpToTask": "Zur Aufgabe springen",