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

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

src/ClaudeDo.Worker/Runner/WorktreeManager.cs

@@ -140,4 +140,32 @@ public sealed class WorktreeManager
         _logger.LogInformation("Committed changes for task {TaskId}: {Head}", task.Id, head);
         return true;
     }
+
+    public async Task DiscardAsync(WorktreeEntity wt, string workingDir, CancellationToken ct)
+    {
+        try
+        {
+            await _git.WorktreeRemoveAsync(workingDir, wt.Path, force: true, ct);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "git worktree remove failed for {Path}", wt.Path);
+            throw;
+        }
+
+        try
+        {
+            await _git.BranchDeleteAsync(workingDir, wt.BranchName, force: true, ct);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogWarning(ex, "git branch -D {Branch} failed after worktree removal; continuing", wt.BranchName);
+        }
+
+        using var context = _dbFactory.CreateDbContext();
+        var wtRepo = new WorktreeRepository(context);
+        await wtRepo.SetStateAsync(wt.TaskId, WorktreeState.Discarded, ct);
+
+        _logger.LogInformation("Discarded worktree for task {TaskId} (branch {Branch})", wt.TaskId, wt.BranchName);
+    }
 }