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/export_test.go

@@ -0,0 +1,11 @@
+package cli
+
+// RootCmd and Execute are rootCmd and execute, exported for the external
+// test package, which builds the command tree itself so that it can
+// supply the arguments and read the output.
+//
+//nolint:gochecknoglobals // test seams, not mutable state.
+var (
+	RootCmd = rootCmd
+	Execute = execute
+)