feat(worker): auto-rebase overlapping WaitingForReview branches after a merge

After a merge lands on the target branch, TaskMergeService now rebases every
other WaitingForReview task's active worktree onto the new tip -- but only
when that branch actually touches a file the merge just changed, computed via
GitService.GetChangedFileNamesAsync (both the merge's landed files and each
candidate's own diff). A branch merely behind is left alone. On conflict or
any other failure the rebase is aborted and the branch is left exactly as it
was, with a WorkerLog warning naming the task; a successful rebase updates the
worktree's recorded BaseCommit/HeadCommit and broadcasts WorktreeUpdated.

src/ClaudeDo.Worker/Lifecycle/TaskMergeService.cs

@@ -161,6 +161,89 @@ public sealed class TaskMergeService
             await _state.ApproveReviewAsync(task.Id, ct);
     }
 
+    /// <summary>
+    /// After a merge lands on <paramref name="targetBranch"/>, rebases every other WaitingForReview
+    /// task's active worktree in the same list onto the new tip -- but only ones that actually
+    /// touch a file the merge just changed. A branch merely behind (no overlap) is left alone: a
+    /// rebase is disruptive busywork when there's nothing to gain from it. Best-effort throughout --
+    /// this runs after the merge that matters has already succeeded, so a failure here must never
+    /// surface as a failure of that merge.
+    /// </summary>
+    private async Task RebaseOthersAfterMergeAsync(
+        TaskEntity mergedTask, ListEntity list, string targetBranch,
+        string oldTargetTip, string newTargetTip, CancellationToken ct)
+    {
+        if (string.Equals(oldTargetTip, newTargetTip, StringComparison.Ordinal)) return;
+
+        IReadOnlyList<string> landedFiles;
+        try
+        {
+            landedFiles = await _git.GetChangedFileNamesAsync(list.WorkingDir!, oldTargetTip, newTargetTip, ct);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogWarning(ex, "Auto-rebase: could not compute files landed by merging task {TaskId}", mergedTask.Id);
+            return;
+        }
+        if (landedFiles.Count == 0) return;
+
+        List<TaskEntity> candidates;
+        using (var ctx = _dbFactory.CreateDbContext())
+        {
+            var tasks = await new TaskRepository(ctx).GetByListIdAsync(list.Id, ct);
+            candidates = tasks.Where(t => t.Id != mergedTask.Id && t.Status == TaskStatus.WaitingForReview).ToList();
+        }
+
+        foreach (var candidate in candidates)
+            await RebaseOneIfOverlappingAsync(candidate, targetBranch, newTargetTip, landedFiles, ct);
+    }
+
+    private async Task RebaseOneIfOverlappingAsync(
+        TaskEntity candidate, string targetBranch, string newTargetTip,
+        IReadOnlyList<string> landedFiles, CancellationToken ct)
+    {
+        WorktreeEntity? wt;
+        using (var ctx = _dbFactory.CreateDbContext())
+            wt = await new WorktreeRepository(ctx).GetByTaskIdAsync(candidate.Id, ct);
+
+        if (wt is null || wt.State != WorktreeState.Active || !Directory.Exists(wt.Path)) return;
+
+        IReadOnlyList<string> ownFiles;
+        try
+        {
+            if (await _git.HasChangesAsync(wt.Path, includeUntracked: false, ct)) return;
+            ownFiles = await _git.GetChangedFileNamesAsync(wt.Path, wt.BaseCommit, "HEAD", ct);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogWarning(ex, "Auto-rebase: could not inspect worktree for task {TaskId}", candidate.Id);
+            return;
+        }
+
+        if (!landedFiles.Intersect(ownFiles, StringComparer.OrdinalIgnoreCase).Any()) return; // merely behind
+
+        var (exitCode, stderr, conflictFiles) = await _git.RebaseAsync(wt.Path, targetBranch, ct);
+        if (exitCode != 0)
+        {
+            var detail = conflictFiles.Count > 0 ? $"conflicts in {string.Join(", ", conflictFiles)}" : stderr;
+            _logger.LogWarning(
+                "Auto-rebase of task {TaskId} branch {Branch} onto {Target} failed, left as-is: {Detail}",
+                candidate.Id, wt.BranchName, targetBranch, detail);
+            await _broadcaster.WorkerLog(
+                $"Auto-rebase failed for \"{candidate.Title}\" onto {targetBranch} — left as-is ({detail})",
+                WorkerLogLevel.Warn, DateTime.UtcNow);
+            return;
+        }
+
+        var newHead = await _git.RevParseHeadAsync(wt.Path, ct);
+        using (var ctx = _dbFactory.CreateDbContext())
+            await new WorktreeRepository(ctx).SetRebasedAsync(candidate.Id, newTargetTip, newHead, ct);
+
+        await _broadcaster.WorktreeUpdated(candidate.Id);
+        _logger.LogInformation(
+            "Auto-rebased task {TaskId} branch {Branch} onto {Target}", candidate.Id, wt.BranchName, targetBranch);
+    }
+
     public async Task<MergeResult> MergeAsync(
         string taskId,
         string targetBranch,
@@ -198,6 +281,7 @@ public sealed class TaskMergeService
                 catch (Exception ex) { return Blocked($"failed to switch target branch: {ex.Message}"); }
             }
 
+            var oldTargetTip = await _git.RevParseHeadAsync(list.WorkingDir, ct);
             var (exitCode, stderr) = await _git.MergeNoFfAsync(list.WorkingDir, wt.BranchName, commitMessage, ct);
             if (exitCode != 0)
             {
@@ -251,6 +335,7 @@ public sealed class TaskMergeService
             }
 
             await MarkWorktreeMergedAsync(taskId, mergeSha, ct);
+            await RebaseOthersAfterMergeAsync(task, list, targetBranch, oldTargetTip, mergeSha, ct);
 
             var verifyFailure = await RunVerifyGateAsync(verifyCommand, list.WorkingDir, ct);
             if (verifyFailure is not null)
@@ -295,6 +380,10 @@ public sealed class TaskMergeService
             if (!await _git.IsMidMergeAsync(list.WorkingDir, ct))
                 return Blocked("repo is not mid-merge");
 
+            // HEAD still points at the pre-merge tip here: a conflicted `git merge` never moves it,
+            // only MERGE_HEAD plus the working tree/index change.
+            var oldTargetTip = await _git.RevParseHeadAsync(list.WorkingDir, ct);
+
             // Validate BEFORE staging: `git add` marks a conflicted path resolved regardless of
             // its content, so an unresolved file with markers still in it would otherwise get
             // staged (and committed) as-is. Check text content for markers first; binary files
@@ -327,6 +416,9 @@ public sealed class TaskMergeService
             var mergeSha = await _git.RevParseHeadAsync(list.WorkingDir, ct);
             await MarkWorktreeMergedAsync(taskId, mergeSha, ct);
 
+            var targetBranch = await _git.GetCurrentBranchAsync(list.WorkingDir, ct);
+            await RebaseOthersAfterMergeAsync(task, list, targetBranch, oldTargetTip, mergeSha, ct);
+
             var verifyFailure = await RunVerifyGateAsync(verifyCommand, list.WorkingDir, ct);
             if (verifyFailure is not null)
             {