feat(git): add ff-only merge and allow a merged worktree without merge commit

src/ClaudeDo.Data/Git/GitService.cs

@@ -450,6 +450,14 @@ public sealed class GitService
         return (exitCode, stderr);
     }
 
+    /// <summary><c>git merge --ff-only</c>: moves the checked-out branch to <paramref name="sourceRef"/> or fails without touching it.</summary>
+    public async Task<(int ExitCode, string Stderr)> MergeFastForwardOnlyAsync(
+        string worktreePath, string sourceRef, CancellationToken ct = default)
+    {
+        var (exitCode, _, stderr) = await RunGitAsync(worktreePath, ["merge", "--ff-only", sourceRef], ct);
+        return (exitCode, stderr);
+    }
+
     public async Task MergeAbortAsync(string repoDir, CancellationToken ct = default)
     {
         var (exitCode, _, stderr) = await RunGitAsync(repoDir, ["merge", "--abort"], ct);

src/ClaudeDo.Data/Repositories/WorktreeRepository.cs

@@ -36,7 +36,7 @@ public sealed class WorktreeRepository
             .ExecuteUpdateAsync(s => s.SetProperty(w => w.State, state), ct);
     }
 
-    public async Task SetMergedAsync(string taskId, string mergeCommit, CancellationToken ct = default)
+    public async Task SetMergedAsync(string taskId, string? mergeCommit, CancellationToken ct = default)
     {
         await _context.Worktrees
             .Where(w => w.TaskId == taskId)