fix(merge): refuse a merge that would overwrite an untracked file in the target

Before merging (or staging a conflict resolution's git add -A), compare the
branch's newly-added paths against what's currently untracked in the target
working directory. A collision aborts with a new untracked_collision status
naming the path and size, surfaced through merge_task/review_task,
preview_merge/preview_merge_set (which merge-tree alone can't see), and the
UI merge paths via FlashFooterError/ShowErrorAsync instead of a silent
catch{}. git's own preflight already refuses this while the path stays
untracked at merge time; this closes the gap once a path becomes trackable
in between (e.g. an unrelated conflict resolution's git add -A) or across
the continue_merge TOCTOU window.

src/ClaudeDo.Localization/locales/en.json

@@ -662,7 +662,7 @@
     "tasksIsland": { "completedHeader": "COMPLETED", "completedHeaderCount": "COMPLETED · {0}", "planningOpenFailed": "Couldn't open planning session: {0}", "planningResumeFailed": "Couldn't resume planning session: {0}", "approveFailed": "Approve & merge failed: {0}", "cancelReviewFailed": "Cancel review failed: {0}", "sendToQueueFailed": "Send to queue failed: {0}", "queuePlanBlockedInteractive": "Can't queue the plan — {0} has an open interactive session and must be closed first.", "moveRunningRejected": "Can't move a running task to another list.", "moveWorktreeRejected": "Can't move — this task has an active worktree pointing at its current repo.", "moveRepoConfirm": "Different repos — {0} → {1}. Move the task anyway?", "moveConfirmUnavailable": "Can't move — the confirmation dialog isn't available.", "quickClaudeNoWorkingDir": "This list has no working directory configured.", "quickClaudeDirMissing": "Working directory no longer exists: {0}" },
     "diff": { "loadFailed": "Failed to load diff: {0}", "noChanges": "No changes to show.", "unavailable": "Diff no longer available — commit range incomplete." },
     "planningDiff": { "hubError": "Could not build combined preview (hub error).", "conflict": "Cannot build combined preview: subtask {0} conflicts with an earlier subtask ({1} files).", "buildFailed": "Could not build combined preview: {0}" },
-    "merge": { "commitMessage": "Merge task: {0}", "workerOfflineBranches": "Worker offline — cannot list branches.", "loadBranchesFailed": "Failed to load branches: {0}", "merged": "Merged.", "conflict": "Merge conflict — target branch restored. Resolve manually or via Continue, then retry.", "blocked": "Blocked: {0}", "verifyFailed": "Merge landed, but the list's verify command failed — the task was kept out of Done.", "unknownStatus": "Unknown status: {0}", "mergeFailed": "Merge failed: {0}" },
+    "merge": { "commitMessage": "Merge task: {0}", "workerOfflineBranches": "Worker offline — cannot list branches.", "loadBranchesFailed": "Failed to load branches: {0}", "merged": "Merged.", "conflict": "Merge conflict — target branch restored. Resolve manually or via Continue, then retry.", "blocked": "Blocked: {0}", "verifyFailed": "Merge landed, but the list's verify command failed — the task was kept out of Done.", "untrackedCollision": "Merge refused — it would overwrite an untracked file in the target working directory.", "unknownStatus": "Unknown status: {0}", "mergeFailed": "Merge failed: {0}" },
     "conflictResolution": { "vsCodeError": "Could not launch VS Code: {0}. Paths are listed above — copy them manually.", "subtaskPrefix": "Conflicts in subtask: {0}", "targetPrefix": "Merging into: {0}" },
     "settingsModal": { "workerOffline": "Worker offline — settings read-only.", "saveFailed": "Save failed: {0}" },
     "onlineInbox": { "workerOffline": "Worker offline — cannot load config.", "saved": "Config saved.", "saveFailed": "Save failed: {0}", "signedIn": "Signed in successfully.", "signedInNoRole": "Signed in, but this account is missing the 'user' role in Zitadel — online sync will be rejected until the role is granted in the ClaudeDo project.", "signInFailed": "Sign-in failed: {0}", "signedOut": "Signed out.", "signOutFailed": "Sign-out failed: {0}" },
@@ -677,7 +677,7 @@
     "worktreesTab": { "workerOffline": "Worker offline.", "removed": "Removed {0} worktree(s).", "blocked": "Cannot force-remove: {0} task(s) still running. Cancel them first.", "removedFrom": "Removed {0} worktree(s) from {1} task(s).", "cleanupFailed": "Cleanup failed: {0}", "resetFailed": "Reset failed: {0}" },
     "worktreesOverview": { "titleAll": "Worktrees", "titleList": "Worktrees — {0}", "listFallback": "list", "cleanupFailed": "Cleanup failed.", "cleanupFailedDetailed": "Cleanup failed: {0}", "removed": "Removed {0} worktree(s).", "discardFailed": "Failed to discard worktree.", "keepFailed": "Failed to keep worktree.", "cannotForceRunning": "Cannot force-remove a running task.", "forceRemoveFailed": "Force remove failed.", "forceRemoveFailedDetailed": "Force remove failed: {0}", "batchProgress": "Merging {0}/{1}…", "batchDone": "Merged {0}, {1} need resolution." },
     "listSettings": { "untitled": "Untitled" },
-    "detailsIsland": { "verifyFailed": "Merge landed, but the list's verify command failed — the task was kept out of Done." },
+    "detailsIsland": { "verifyFailed": "Merge landed, but the list's verify command failed — the task was kept out of Done.", "untrackedCollision": "Merge refused — it would overwrite an untracked file in the target working directory." },
     "lists": { "localSuffix": "{0} / local", "smartMyDay": "My Day", "smartImportant": "Important", "smartPlanned": "Planned", "virtualQueue": "Queue", "virtualRunning": "Running", "virtualReview": "Review", "newList": "New list" },
     "repoImport": { "loadFailed": "Couldn't load remembered folders: {0}", "saveFailed": "Couldn't save folders: {0}" }
   },