test: expand coverage across readers, writers, cmd, ui, diff and merge

Implements tests/_plans and previously deferred packages; updates plan
README with new coverage numbers.

pkg/writers/template/errors_test.go

@@ -0,0 +1,48 @@
+package template
+
+import (
+	"errors"
+	"strings"
+	"testing"
+)
+
+func TestTemplateError(t *testing.T) {
+	cause := errors.New("boom")
+	tests := []struct {
+		name  string
+		err   *TemplateError
+		phase string
+	}{
+		{"load", NewTemplateLoadError("cannot read", cause), "load"},
+		{"parse", NewTemplateParseError("bad syntax", cause), "parse"},
+		{"execute", NewTemplateExecuteError("failed render", cause), "execute"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if tt.err.Phase != tt.phase {
+				t.Errorf("phase = %q", tt.err.Phase)
+			}
+			msg := tt.err.Error()
+			if !strings.Contains(msg, "template "+tt.phase+" error") || !strings.Contains(msg, "boom") {
+				t.Errorf("message = %q", msg)
+			}
+			if !errors.Is(tt.err, cause) {
+				t.Error("errors.Is must reach cause")
+			}
+			var te *TemplateError
+			if !errors.As(error(tt.err), &te) || te != tt.err {
+				t.Error("errors.As failed")
+			}
+		})
+	}
+}
+
+func TestTemplateErrorWithoutCause(t *testing.T) {
+	e := NewTemplateParseError("only message", nil)
+	if got := e.Error(); got != "template parse error: only message" {
+		t.Errorf("got %q", got)
+	}
+	if e.Unwrap() != nil {
+		t.Error("Unwrap must be nil")
+	}
+}