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.Data/Git/GitService.cs

@@ -160,6 +160,13 @@ public sealed class GitService
         return stdout.Trim();
     }
 
+    public async Task CheckoutBranchAsync(string repoDir, string branchName, CancellationToken ct = default)
+    {
+        var (exitCode, _, stderr) = await RunGitAsync(repoDir, ["checkout", branchName], ct);
+        if (exitCode != 0)
+            throw new InvalidOperationException($"git checkout '{branchName}' failed (exit {exitCode}): {stderr}");
+    }
+
     public async Task<List<string>> ListLocalBranchesAsync(string repoDir, CancellationToken ct = default)
     {
         var (exitCode, stdout, stderr) = await RunGitAsync(repoDir,