Fix multiple code review issues: dedup splitKey, fix merge bug, remove dead code

- Extract triplicated splitKey into shared pkg/reconciler/internal package
  using strings.Cut and removing the unnecessary error return
- Fix resolver merge bug: attemptMerge was using treeA.Hash as the merge
  commit tree, silently dropping all of branch B's changes. Now builds a
  proper merged tree that applies B's non-conflicting changes onto A's tree
- Replace hand-rolled nestedFieldNoCopy/unstructuredNestedStringMap with
  k8s.io/apimachinery's unstructured.NestedStringMap stdlib equivalent
- Hoist branch listing out of per-refspec loop in updateBranches to avoid
  redundant API calls
- Add not-found handling in all three reconcilers so deleted resources
  don't cause infinite requeue loops
- Fix client.Get() to use checked type assertion with a clear panic message
  instead of an opaque nil-pointer dereference
- Remove dead code: unused addDefaultingFuncs wrapper, unused context
  injection in push controller setup

https://claude.ai/code/session_01634JRpHEoM9FzuBjZ7qHcY

pkg/reconciler/push/controller.go

@@ -35,7 +35,6 @@ func NewController(ctx context.Context, cmw configmap.Watcher) *controller.Impl
 	}
 
 	gitClient := gitclient.NewFromDynamic(dynClient)
-	ctx = gitclient.WithClient(ctx, gitClient)
 
 	// Create dynamic informer for GitPushTransaction.
 	factory := dynamicinformer.NewDynamicSharedInformerFactory(dynClient, 30*time.Second)