feat(worker): implement TaskMergeService happy path

src/ClaudeDo.Worker/Services/TaskMergeService.cs

@@ -75,8 +75,55 @@ public sealed class TaskMergeService
         if (await _git.HasChangesAsync(list.WorkingDir, ct))
             return Blocked("target working tree has uncommitted changes");
 
-        // Body added in later tasks.
-        throw new NotImplementedException();
+        var (exitCode, stderr) = await _git.MergeNoFfAsync(list.WorkingDir, wt.BranchName, commitMessage, ct);
+        if (exitCode != 0)
+        {
+            List<string> files;
+            try { files = await _git.ListConflictedFilesAsync(list.WorkingDir, ct); }
+            catch { files = new(); }
+            try { await _git.MergeAbortAsync(list.WorkingDir, ct); }
+            catch (Exception ex) { _logger.LogWarning(ex, "git merge --abort failed after conflict"); }
+
+            if (files.Count == 0)
+            {
+                // Non-conflict failure (e.g. unrelated histories).
+                return new MergeResult(StatusBlocked, Array.Empty<string>(), $"merge failed: {stderr}");
+            }
+
+            return new MergeResult(StatusConflict, files, null);
+        }
+
+        string? cleanupWarning = null;
+        if (removeWorktree)
+        {
+            try
+            {
+                await _git.WorktreeRemoveAsync(list.WorkingDir, wt.Path, force: false, ct);
+                try { await _git.BranchDeleteAsync(list.WorkingDir, wt.BranchName, force: false, ct); }
+                catch (Exception ex)
+                {
+                    _logger.LogWarning(ex, "branch delete failed for {Branch}", wt.BranchName);
+                    cleanupWarning = $"worktree removed, branch delete failed: {ex.Message}";
+                }
+            }
+            catch (Exception ex)
+            {
+                _logger.LogWarning(ex, "worktree remove failed for {Path}", wt.Path);
+                cleanupWarning = $"worktree remove failed: {ex.Message}";
+            }
+        }
+
+        using (var ctx = _dbFactory.CreateDbContext())
+        {
+            await new WorktreeRepository(ctx).SetStateAsync(taskId, WorktreeState.Merged, ct);
+        }
+        await _broadcaster.WorktreeUpdated(taskId);
+
+        _logger.LogInformation(
+            "Merged task {TaskId} branch {Branch} into {Target} (remove worktree: {Remove})",
+            taskId, wt.BranchName, targetBranch, removeWorktree);
+
+        return new MergeResult(StatusMerged, Array.Empty<string>(), cleanupWarning);
     }
 
     public async Task<MergeTargets> GetTargetsAsync(string taskId, CancellationToken ct)