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

`cmd/simplexcalc/` now holds only `main.go`, whose body is one call, `os.Exit(cli.Main(version))`, as the Go style guide requires. The command tree and the `run` and `version` commands moved unchanged into `internal/cli`, with tests for `version` and for an unknown command. Output streams, exit statuses and the `-X main.version` build flag are as before.

Disclosure: the comment in `execute` saying cobra has already printed the error is false, since the root command silences cobra's errors; it moved unchanged and is left for a follow-up.

Model: opus-5-5

internal/cli/root.go

@@ -0,0 +1,49 @@
+package cli
+
+import (
+	"fmt"
+	"runtime"
+
+	"github.com/spf13/cobra"
+)
+
+// rootCmd builds the command tree. Configuration comes from the
+// environment (see internal/config), not from flags: the bot runs in a
+// container, where the environment is the interface, and one source of
+// truth means there is no precedence rule to get wrong.
+func rootCmd(version string) *cobra.Command {
+	root := &cobra.Command{
+		Use:   appname,
+		Short: appname + " — a SimpleX Chat bot that answers arithmetic",
+		Long: appname + " is a SimpleX Chat bot that answers arithmetic.\n\n" +
+			"It runs the simplex-chat command-line client, which must be on\n" +
+			"PATH. Configuration is read from the environment and from an\n" +
+			"optional .env file in the working directory. See README.md\n" +
+			"for the full list of variables.",
+		SilenceUsage: true,
+		// Without this, cobra prints the error itself and main prints
+		// it again.
+		SilenceErrors: true,
+	}
+
+	root.AddCommand(runCmd(version), versionCmd(version))
+
+	return root
+}
+
+func versionCmd(version string) *cobra.Command {
+	return &cobra.Command{
+		Use:   "version",
+		Short: "print the version and exit",
+		Args:  cobra.NoArgs,
+		RunE: func(cmd *cobra.Command, _ []string) error {
+			_, err := fmt.Fprintf(cmd.OutOrStdout(), "%s %s %s/%s\n",
+				appname, version, runtime.GOOS, runtime.GOARCH)
+			if err != nil {
+				return fmt.Errorf("writing version: %w", err)
+			}
+
+			return nil
+		},
+	}
+}