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/which.md

@@ -0,0 +1,31 @@
+---
+name: which
+kind: function
+lang: go
+domain: shell
+version: "1.0.0"
+purity: pure
+signature: "func Which(name string) core.Option[string]"
+description: "Busca la ruta de un ejecutable en el PATH del sistema. Devuelve None si no existe."
+tags: [shell, path, lookup]
+uses_functions: []
+uses_types: [option_go_core]
+returns: [option_go_core]
+returns_optional: false
+error_type: ""
+imports: []
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/shell/which.go"
+---
+
+## Ejemplo
+
+```go
+path := Which("docker")
+```
+
+## Notas
+
+Implementacion en devfactory/shell.