feat(worker): use the list base branch as worktree base and default merge target

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

@@ -425,6 +425,30 @@ public class WorktreeManagerTests : IDisposable
         Assert.StartsWith("chore/77-test-task", ctx.BranchName);
     }
 
+    [Fact]
+    public async Task CreateAsync_BaseBranchSet_BasesOnItEvenWhenAnotherBranchIsCheckedOut()
+    {
+        if (!GitAvailable) { Assert.True(true, "git not available -- skipping"); return; }
+
+        var repo = CreateRepo();
+        GitRepoFixture.RunGit(repo.RepoDir, "checkout", "-b", "dev");
+        File.WriteAllText(Path.Combine(repo.RepoDir, "dev.txt"), "dev");
+        GitRepoFixture.RunGit(repo.RepoDir, "add", "-A");
+        GitRepoFixture.RunGit(repo.RepoDir, "commit", "-m", "dev work");
+        var devHead = GitRepoFixture.RunGit(repo.RepoDir, "rev-parse", "HEAD").Trim();
+        GitRepoFixture.RunGit(repo.RepoDir, "checkout", "-b", "elsewhere", repo.BaseCommit);
+
+        var (task, list) = MakeEntities(repo.RepoDir);
+        var (mgr, db) = await CreateManagerAsync(task, list);
+        using (var ctx0 = db.CreateContext())
+            await new ListRepository(ctx0).SetConfigAsync(new ListConfigEntity { ListId = list.Id, BaseBranch = "dev" });
+
+        var ctx = await mgr.CreateAsync(task, list, CancellationToken.None);
+        _worktreeCleanups.Add((repo.RepoDir, ctx.WorktreePath));
+
+        Assert.Equal(devHead, ctx.BaseCommit);
+    }
+
     private static (TaskEntity task, ListEntity list) MakeEntities(string workingDir)
     {
         var listId = Guid.NewGuid().ToString();