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

agent/test/proxmox_agent/shell_test.exs

@@ -0,0 +1,19 @@
+defmodule ProxmoxAgent.ShellTest do
+  use ExUnit.Case, async: true
+
+  alias ProxmoxAgent.Shell
+
+  test "run/2 returns {:ok, output} on zero exit" do
+    assert {:ok, output} = Shell.run("/bin/echo", ["hello"])
+    assert String.trim(output) == "hello"
+  end
+
+  test "run/2 returns {:error, {:nonzero_exit, code, output}} on non-zero exit" do
+    assert {:error, {:nonzero_exit, code, _}} = Shell.run("/bin/sh", ["-c", "exit 7"])
+    assert code == 7
+  end
+
+  test "run/2 returns {:error, {:enoent, _}} when binary is missing" do
+    assert {:error, {:enoent, _}} = Shell.run("/does/not/exist/nope", [])
+  end
+end