feat(agent): Shell.run wrapper for testable external commands

agent/lib/proxmox_agent/shell.ex

@@ -0,0 +1,24 @@
+defmodule ProxmoxAgent.Shell do
+  @moduledoc """
+  Thin wrapper over System.cmd for testability. Collectors accept an optional
+  :runner function of this shape so tests can inject fixture-backed fakes.
+  """
+
+  @type result :: {:ok, String.t()} | {:error, term()}
+
+  @spec run(String.t(), [String.t()]) :: result
+  def run(command, args) do
+    try do
+      case System.cmd(command, args, stderr_to_stdout: true) do
+        {output, 0} -> {:ok, output}
+        {output, code} -> {:error, {:nonzero_exit, code, output}}
+      end
+    rescue
+      e in ErlangError ->
+        case e.original do
+          :enoent -> {:error, {:enoent, command}}
+          other -> {:error, {:system_error, other}}
+        end
+    end
+  end
+end