fix(worktree): auto-bootstrap commit-less repos before creating worktrees

Covers autonomous runs, interactive ConPTY sessions (GetInteractiveLaunchSpec),
planning sessions, and improvement/planning children — every path that needs a
base commit now self-heals on a fresh 'git init' repo instead of surfacing
"ambiguous argument 'HEAD'" as a HubException.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
(cherry picked from commit ec16a85c495ceceaa22f922e077a27dcda54f035)

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

@@ -191,6 +191,34 @@ public class WorktreeManagerTests : IDisposable
         Assert.Contains("hello.txt", row.DiffStat);
     }
 
+    [Fact]
+    public async Task CreateAsync_EmptyRepo_BootstrapsInitialCommit()
+    {
+        if (!GitAvailable) { Assert.True(true, "git not available -- skipping"); return; }
+
+        // Fresh `git init`, zero commits, no identity configured — the cross-PC failure
+        // that surfaced as "git rev-parse HEAD failed: ambiguous argument 'HEAD'".
+        var repoDir = Path.Combine(Path.GetTempPath(), $"claudedo_emptyrepo_{Guid.NewGuid():N}");
+        Directory.CreateDirectory(repoDir);
+        GitRepoFixture.RunGit(repoDir, "init", "-b", "main");
+        _tempDirs.Add(repoDir);
+
+        var (task, list) = MakeEntities(repoDir);
+        var (mgr, db) = await CreateManagerAsync(task, list);
+
+        var ctx = await mgr.CreateAsync(task, list, CancellationToken.None);
+        _worktreeCleanups.Add((repoDir, ctx.WorktreePath));
+
+        Assert.True(Directory.Exists(ctx.WorktreePath));
+        // The base commit is the auto-created bootstrap commit.
+        var head = GitRepoFixture.RunGit(repoDir, "rev-parse", "HEAD").Trim();
+        Assert.Equal(head, ctx.BaseCommit);
+
+        using var readCtx = db.CreateContext();
+        var row = await new WorktreeRepository(readCtx).GetByTaskIdAsync(task.Id);
+        Assert.Equal(WorktreeState.Active, row!.State);
+    }
+
     [Fact]
     public async Task CreateAsync_NonGitDir_Throws_NoRow()
     {