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/export_test.go

@@ -0,0 +1,7 @@
+package cli
+
+// Execute is execute, exported for the external test package, which
+// needs to supply the arguments and read the output.
+//
+//nolint:gochecknoglobals // a test seam, not mutable state.
+var Execute = execute