feat: tipo CmdResult y 6 funciones shell — ejecucion de comandos del sistema

Tipo cmd_result (product: Stdout, Stderr, ExitCode). Funciones: Run,
RunWithTimeout, RunShell, RunShellTimeout, RunPipe (impuras) y Which (pura).
Stubs que documentan devfactory/shell para el registry.

functions/shell/run_shell.go

@@ -0,0 +1,9 @@
+package shell
+
+// Implementation: github.com/lucasdataproyects/devfactory/shell
+
+// RunShell ejecuta un comando shell interpretado por /bin/sh.
+func RunShell(command string) Result[CmdResult] {
+	// stub — implementation in devfactory/shell
+	return Result[CmdResult]{}
+}