Improve test coverage from 10.3% to 42.7% with diffcover analysis

Apply the diffcover technique to map code snippets to features, identify
under-tested components, and add comprehensive tests for all reconciler
business logic. Key improvements:

- pkg/reconciler/internal: 26.7% → 100% (generic adapter fully tested)
- pkg/client: 5.6% → 71.8% (full CRUD for all 4 resource types)
- pkg/reconciler/repowatcher: 10.1% → 56.3% (ReconcileKind lifecycle)
- pkg/reconciler/push: 0% → 33.3% (failTransaction, updateBranches)
- pkg/reconciler/sync: 0% → 33.0% (findBranch, createPushTransaction)
- pkg/reconciler/resolver: 2.1% → 17.0% (merge transactions, manual intervention)

Includes COVERAGE_REPORT.md with full code-to-feature mapping showing
which tests cover which lines, following the diffcover methodology.

https://claude.ai/code/session_016Yeyfoocn7SN2HuMsxRD6f

pkg/apis/git/v1alpha1/defaults_test.go

@@ -2,6 +2,8 @@ package v1alpha1
 
 import (
 	"testing"
+
+	"k8s.io/apimachinery/pkg/runtime"
 )
 
 func TestSetDefaults_GitRepository(t *testing.T) {
@@ -67,3 +69,31 @@ func TestSetDefaults_GitPushTransaction(t *testing.T) {
 		})
 	}
 }
+
+func TestRegisterDefaults(t *testing.T) {
+	scheme := runtime.NewScheme()
+
+	// Register the types so the scheme knows about them.
+	if err := AddToScheme(scheme); err != nil {
+		t.Fatalf("AddToScheme() error = %v", err)
+	}
+
+	// Register defaults.
+	if err := RegisterDefaults(scheme); err != nil {
+		t.Fatalf("RegisterDefaults() error = %v", err)
+	}
+
+	// Verify GitRepository defaults are applied via scheme.
+	repo := &GitRepository{}
+	scheme.Default(repo)
+	if repo.Spec.DefaultBranch != "main" {
+		t.Errorf("DefaultBranch after scheme.Default = %q, want %q", repo.Spec.DefaultBranch, "main")
+	}
+
+	// Verify GitPushTransaction defaults are applied via scheme.
+	txn := &GitPushTransaction{}
+	scheme.Default(txn)
+	if txn.Status.Phase != TransactionPhasePending {
+		t.Errorf("Phase after scheme.Default = %q, want %q", txn.Status.Phase, TransactionPhasePending)
+	}
+}