Move the command tree into internal/cli (closes #8)

cmd/simplexcalc/ now holds only main.go: it passes the version the
linker sets to cli.Main and exits with the status that returns. root.go
and run.go move to internal/cli with their comments, changed only to
take the version as a parameter. Main reads the process's arguments and
output; execute takes them as parameters so the new tests can check the
version line and the non-zero status of an unknown command. Commands,
flags, output, log lines and exit status are unchanged, and the
-X main.version flag in the Dockerfile still applies.

Model: opus-5-5

internal/cli/cli_test.go

@@ -0,0 +1,41 @@
+package cli_test
+
+import (
+	"bytes"
+	"runtime"
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/simplexcalc/internal/cli"
+)
+
+func TestVersionPrintsVersion(t *testing.T) {
+	t.Parallel()
+
+	var stdout, stderr bytes.Buffer
+
+	status := cli.Execute("1.2.3", []string{"version"}, &stdout, &stderr)
+	if status != 0 {
+		t.Fatalf("status = %d, want 0; stderr: %q", status, stderr.String())
+	}
+
+	want := "simplexcalc 1.2.3 " + runtime.GOOS + "/" + runtime.GOARCH + "\n"
+	if stdout.String() != want {
+		t.Errorf("stdout = %q, want %q", stdout.String(), want)
+	}
+}
+
+func TestUnknownCommandFails(t *testing.T) {
+	t.Parallel()
+
+	var stdout, stderr bytes.Buffer
+
+	status := cli.Execute("1.2.3", []string{"frobnicate"}, &stdout, &stderr)
+	if status == 0 {
+		t.Fatal("status = 0, want non-zero")
+	}
+
+	if !strings.HasPrefix(stderr.String(), "exiting: ") {
+		t.Errorf("stderr = %q, want the error after \"exiting: \"", stderr.String())
+	}
+}