feat(worker): add WorktreeManager.DiscardAsync for task reset

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

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

@@ -181,6 +181,32 @@ public class WorktreeManagerTests : IDisposable
         return (task, list);
     }
 
+    [Fact]
+    public async Task DiscardAsync_RemovesWorktreeAndBranch_AndSetsStateDiscarded()
+    {
+        if (!GitAvailable) { Assert.True(true, "git not available -- skipping"); return; }
+
+        var repo = CreateRepo();
+        var (task, list) = MakeEntities(repo.RepoDir);
+        var (mgr, db) = await CreateManagerAsync(task, list);
+
+        var ctx = await mgr.CreateAsync(task, list, CancellationToken.None);
+        var worktreePath = ctx.WorktreePath;
+
+        WorktreeEntity wt;
+        using (var readCtx = db.CreateContext())
+            wt = (await new WorktreeRepository(readCtx).GetByTaskIdAsync(task.Id))!;
+
+        await mgr.DiscardAsync(wt, list.WorkingDir!, CancellationToken.None);
+
+        Assert.False(Directory.Exists(worktreePath), "worktree directory should be gone");
+
+        using var readCtx2 = db.CreateContext();
+        var row = await new WorktreeRepository(readCtx2).GetByTaskIdAsync(task.Id);
+        Assert.NotNull(row);
+        Assert.Equal(WorktreeState.Discarded, row!.State);
+    }
+
     public void Dispose()
     {
         foreach (var (repoDir, wtPath) in _worktreeCleanups)