fix(worker): reorder PlanningAggregator checkout/delete and kill git on cancel

Also stub new IWorkerClient planning members in FakeWorkerClient to restore build.

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

src/ClaudeDo.Worker/Planning/PlanningAggregator.cs

@@ -78,11 +78,13 @@ public sealed class PlanningAggregator
 
         var integrationBranch = BuildIntegrationBranchName(planning);
 
-        // Reset: delete if exists, then recreate off the target branch.
+        // Reset: checkout target first (so we're never ON the integration branch when deleting it),
+        // then delete if exists, then recreate off the target branch.
+        await _git.CheckoutBranchAsync(repoDir, targetBranch, ct);
+
         try { await _git.BranchDeleteAsync(repoDir, integrationBranch, force: true, ct); }
         catch { /* didn't exist */ }
 
-        await _git.CheckoutBranchAsync(repoDir, targetBranch, ct);
         await GitRawAsync(repoDir, ct, "checkout", "-b", integrationBranch);
 
         foreach (var child in childSubtasks)
@@ -150,7 +152,15 @@ public sealed class PlanningAggregator
         using var p = System.Diagnostics.Process.Start(psi)!;
         var stdoutTask = p.StandardOutput.ReadToEndAsync();
         var stderrTask = p.StandardError.ReadToEndAsync();
-        await p.WaitForExitAsync(ct);
+        try
+        {
+            await p.WaitForExitAsync(ct);
+        }
+        catch (OperationCanceledException)
+        {
+            try { if (!p.HasExited) p.Kill(entireProcessTree: true); } catch { }
+            throw;
+        }
         var stdout = await stdoutTask;
         var stderr = await stderrTask;
         if (p.ExitCode != 0) throw new InvalidOperationException($"git {string.Join(' ', args)} failed: {stderr}");