feat(worker): add preview_merge and preview_merge_set MCP tools

Give an autonomous reviewer a non-destructive merge-tree preview
(status/conflicts/changedFileCount/behind) for a task's worktree branch,
plus a file-overlap check across a batch of tasks so same-file collisions
between sibling branches are visible before merging. MergeHelperDefault's
Phase 4 now calls preview_merge_set before merging a batch.

src/ClaudeDo.Data/PromptFiles.cs

@@ -273,14 +273,18 @@ public static class PromptFiles
         Poll get_task until every task has left Queued and Running — WaitingForReview on success, Failed on error. Report progress as tasks land; do not poll silently for minutes.
 
         ## Phase 4 — Review and merge
-        One task at a time, in the order the brief lists them.
+        Before merging anything, call preview_merge_set with every surviving task's id (the same targetBranch you are about to merge into). It tells you, per task, whether a clean merge-tree preview is even possible (status/conflictFiles/changedFileCount/behind) and which files more than one of the tasks changed (overlaps). Read the overlaps: a file two tasks both touch is where a same-branch collision could happen. This is a HINT, not proof — it only catches same-file overlap, not a cross-file break (e.g. one task deletes a symbol another task's file still references), and a clean preview never guarantees the result compiles or passes tests. Use it to decide merge order and to know which pairs to look at extra carefully in step 1 below; it does not replace reading the diffs.
 
-        1. Inspect the change with get_task_diff (stat first, then the full diff if it is non-trivial) and sanity-check it against the task's title and description.
+        One task at a time, in the order the brief lists them (adjust the order if the overlap check suggests a safer sequence).
+
+        1. Inspect the change with get_task_diff (stat first, then the full diff if it is non-trivial) and sanity-check it against the task's title and description. If preview_merge_set flagged this task in an overlap, also skim the diff of the other task(s) sharing that file.
         2. If the change looks wrong, incomplete, or risky, STOP and ask the user before merging — offer reject_rerun (with feedback) or skip.
         3. Otherwise merge with review_task(taskId, decision="approve", leaveConflictsInTree=true).
            - Clean merge → the task is Done; move on.
            - Conflict (markers left in the working tree, repoPath returned) → resolve it.
 
+        After each merge, main has moved — a clean preview_merge_set result from before this merge is now stale for the remaining tasks. If you are unsure whether an earlier overlap warning still matters, call preview_merge for the next task again before merging it.
+
         Every branch in this run forked from the same base, so conflicts between them are the NORMAL case, not a failure. Resolve them and keep going; do not abandon the run because a merge conflicted.
 
         Resolving a conflict: