fix(worker): honour targetBranch in MergeAsync by checking out before merge

Add GitService.CheckoutBranchAsync; compare targetBranch to current HEAD
before MergeNoFfAsync and switch when they differ. Returns Blocked if the
branch does not exist. Add three new tests (two service, one GitService).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Worker/Services/TaskMergeService.cs

@@ -75,6 +75,13 @@ public sealed class TaskMergeService
         if (await _git.HasChangesAsync(list.WorkingDir, ct))
             return Blocked("target working tree has uncommitted changes");
 
+        var currentBranch = await _git.GetCurrentBranchAsync(list.WorkingDir, ct);
+        if (!string.Equals(currentBranch, targetBranch, StringComparison.Ordinal))
+        {
+            try { await _git.CheckoutBranchAsync(list.WorkingDir, targetBranch, ct); }
+            catch (Exception ex) { return Blocked($"failed to switch target branch: {ex.Message}"); }
+        }
+
         var (exitCode, stderr) = await _git.MergeNoFfAsync(list.WorkingDir, wt.BranchName, commitMessage, ct);
         if (exitCode != 0)
         {