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.go

@@ -0,0 +1,38 @@
+// Package cli is the simplexcalc command tree: the run and version
+// commands, and the exit status the program ends with.
+package cli
+
+import (
+	"fmt"
+	"io"
+	"os"
+)
+
+// appname is the name the program reports in its logs and its version.
+const appname = "simplexcalc"
+
+// Main runs the command named on the command line and returns the exit
+// status for main to pass to os.Exit.
+func Main(version string) int {
+	return execute(version, os.Args[1:], os.Stdout, os.Stderr)
+}
+
+// execute is Main with the arguments and the output passed in, so that
+// a test can supply them.
+func execute(version string, args []string, stdout, stderr io.Writer) int {
+	root := rootCmd(version)
+	root.SetArgs(args)
+	root.SetOut(stdout)
+	root.SetErr(stderr)
+
+	err := root.Execute()
+	if err != nil {
+		// cobra has already printed the error; this only sets the exit
+		// status, which is what a supervisor and a shell script read.
+		_, _ = fmt.Fprintln(stderr, "exiting: "+err.Error())
+
+		return 1
+	}
+
+	return 0
+}