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>

tests/ClaudeDo.Worker.Tests/Runner/GitServiceMergeTests.cs

@@ -155,6 +155,20 @@ public class GitServiceMergeTests : IDisposable
         Assert.False(await git.IsMidMergeAsync(repo.RepoDir));
     }
 
+    [Fact]
+    public async Task CheckoutBranchAsync_ExistingBranch_SwitchesBranch()
+    {
+        if (!GitRepoFixture.IsGitAvailable()) return;
+        var repo = NewRepo();
+        GitRepoFixture.RunGit(repo.RepoDir, "branch", "feature/checkout-test");
+
+        var git = new GitService();
+        await git.CheckoutBranchAsync(repo.RepoDir, "feature/checkout-test");
+
+        var current = await git.GetCurrentBranchAsync(repo.RepoDir);
+        Assert.Equal("feature/checkout-test", current);
+    }
+
     [Fact]
     public async Task ListConflictedFilesAsync_MidConflict_ReturnsConflictedFile()
     {