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_timeout.md

@@ -0,0 +1,31 @@
+---
+name: run_shell_timeout
+kind: function
+lang: go
+domain: shell
+version: "1.0.0"
+purity: impure
+signature: "func RunShellTimeout(command string, timeout time.Duration) core.Result[CmdResult]"
+description: "Ejecuta un comando shell con timeout configurable."
+tags: [shell, command, process, exec, timeout]
+uses_functions: []
+uses_types: [cmd_result_go_shell, result_go_core]
+returns: [result_go_core]
+returns_optional: false
+error_type: "error_go_core"
+imports: [time]
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/shell/run_shell_timeout.go"
+---
+
+## Ejemplo
+
+```go
+result := RunShellTimeout("long-running-cmd", 30*time.Second)
+```
+
+## Notas
+
+Implementacion en devfactory/shell.