fix(diff): merge template records sharing name+type into one RRset

Two template rows at the same name and type (e.g. two MX) produced two
diffs with one key; each PATCH replaced the whole provider RRset, so only
the last value survived. Merge values by key before diffing.

docs: add lint/single-test commands and compose files to CLAUDE.md

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/diff/diff_test.go

@@ -161,9 +161,9 @@ func TestPrunesEmptyTemplateAllManagedNoneInUpdates(t *testing.T) {
 // ReadOnly NS diff (here an Add) is excluded from both — mirroring Actionable().
 func TestUpdatesAndPrunesMixedSeparation(t *testing.T) {
 	tmpl := []model.Record{
-		{Type: model.A, Name: "a.example.com.", TTL: 300, Values: []string{"1.1.1.1"}}, // in sync
-		{Type: model.A, Name: "b.example.com.", TTL: 300, Values: []string{"9.9.9.9"}}, // update
-		{Type: model.A, Name: "c.example.com.", TTL: 300, Values: []string{"3.3.3.3"}}, // add
+		{Type: model.A, Name: "a.example.com.", TTL: 300, Values: []string{"1.1.1.1"}},          // in sync
+		{Type: model.A, Name: "b.example.com.", TTL: 300, Values: []string{"9.9.9.9"}},          // update
+		{Type: model.A, Name: "c.example.com.", TTL: 300, Values: []string{"3.3.3.3"}},          // add
 		{Type: model.NS, Name: "example.com.", TTL: 3600, Values: []string{"ns1.example.com."}}, // read-only add
 	}
 	actual := []model.Record{
@@ -301,3 +301,24 @@ func TestMarkCustomNeverMarksReadOnly(t *testing.T) {
 		t.Fatalf("read-only diffs must never become custom, got %+v", got)
 	}
 }
+
+// Two template rows at the same name+type are one RRset: values are merged.
+func TestDiffMergesTemplateRecordsWithSameKey(t *testing.T) {
+	tpl := []model.Record{
+		{Type: model.MX, Name: "example.com.", TTL: 600, Values: []string{"0 pmg2-mail.example.com."}},
+		{Type: model.MX, Name: "example.com.", TTL: 600, Values: []string{"10 pmg1-mail.example.com."}},
+		{Type: model.MX, Name: "Example.com.", TTL: 600, Values: []string{"10 pmg1-mail.example.com"}}, // dup value
+	}
+	cs := Diff(tpl, nil)
+	if len(cs.Diffs) != 1 {
+		t.Fatalf("want 1 diff, got %d", len(cs.Diffs))
+	}
+	d := cs.Diffs[0]
+	if d.Kind != Add || len(d.Desired.Values) != 2 {
+		t.Fatalf("want Add with 2 values, got %v %v", d.Kind, d.Desired.Values)
+	}
+	actual := []model.Record{{Type: model.MX, Name: "example.com.", TTL: 600, Values: []string{"10 pmg1-mail.example.com.", "0 pmg2-mail.example.com."}}}
+	if got := Diff(tpl, actual).Diffs[0].Kind; got != InSync {
+		t.Fatalf("want InSync, got %v", got)
+	}
+}