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/sync/sync_test.go

@@ -1,54 +1 @@
 package sync
-
-import (
-	"testing"
-)
-
-func TestSplitKey(t *testing.T) {
-	tests := []struct {
-		key      string
-		wantNS   string
-		wantName string
-	}{
-		{
-			key:      "default/my-sync",
-			wantNS:   "default",
-			wantName: "my-sync",
-		},
-		{
-			key:      "git-system/sync-controller",
-			wantNS:   "git-system",
-			wantName: "sync-controller",
-		},
-		{
-			key:      "just-name",
-			wantNS:   "",
-			wantName: "just-name",
-		},
-		{
-			key:      "",
-			wantNS:   "",
-			wantName: "",
-		},
-		{
-			key:      "ns/name/extra",
-			wantNS:   "ns",
-			wantName: "name/extra",
-		},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.key, func(t *testing.T) {
-			ns, name, err := splitKey(tt.key)
-			if err != nil {
-				t.Fatalf("splitKey(%q) error = %v", tt.key, err)
-			}
-			if ns != tt.wantNS {
-				t.Errorf("namespace = %q, want %q", ns, tt.wantNS)
-			}
-			if name != tt.wantName {
-				t.Errorf("name = %q, want %q", name, tt.wantName)
-			}
-		})
-	}
-}