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/apis/git/v1alpha1/defaults.go

@@ -16,13 +16,7 @@ func SetDefaults_GitPushTransaction(t *GitPushTransaction) {
 	}
 }
 
-func addDefaultingFuncs(scheme *runtime.Scheme) error {
-	return RegisterDefaults(scheme)
-}
-
 // RegisterDefaults adds default functions to the given scheme.
-// This is a placeholder that will be populated by code generation.
-// For now we register defaults manually.
 func RegisterDefaults(scheme *runtime.Scheme) error {
 	scheme.AddTypeDefaultingFunc(&GitRepository{}, func(obj interface{}) {
 		SetDefaults_GitRepository(obj.(*GitRepository))