fix(worker): validate conflict markers before staging in ContinueMergeAsync

git add -A resolves the index U-stage regardless of file content, so a
conflicted file whose markers were never removed still got staged and
committed as-is. Check previously-conflicted files for leftover
<<<<<<< markers before staging; refuse with a conflict result if any
remain, leaving the repo mid-merge so Abort still cleans up.

Adds a regression test that leaves one conflict unresolved and asserts
ContinueMergeAsync refuses (no commit) and AbortMergeAsync still
restores a clean tree.

src/ClaudeDo.Worker/Lifecycle/TaskMergeService.cs

@@ -204,6 +204,26 @@ public sealed class TaskMergeService
         if (!await _git.IsMidMergeAsync(list.WorkingDir, ct))
             return Blocked("repo is not mid-merge");
 
+        // 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
+        // can't carry markers, so they're left to the post-stage index check below.
+        var unresolved = await _git.ListConflictedFilesAsync(list.WorkingDir, ct);
+        var stillConflicted = new List<string>();
+        foreach (var path in unresolved)
+        {
+            var full = Path.Combine(list.WorkingDir, path.Replace('/', Path.DirectorySeparatorChar));
+            string text;
+            try { text = await File.ReadAllTextAsync(full, ct); }
+            catch { continue; }
+
+            if (!LooksBinary(text) && ConflictMarkerParser.HasConflicts(text))
+                stillConflicted.Add(path);
+        }
+
+        if (stillConflicted.Count > 0)
+            return new MergeResult(StatusConflict, stillConflicted, "conflicts not fully resolved");
+
         await _git.AddAllAsync(list.WorkingDir, ct);
 
         var remaining = await _git.ListConflictedFilesAsync(list.WorkingDir, ct);