feat(agent): expose compile-time version

agent/test/proxmox_agent_test.exs

@@ -1,8 +1,8 @@
 defmodule ProxmoxAgentTest do
-  use ExUnit.Case
-  doctest ProxmoxAgent
+  use ExUnit.Case, async: true
 
-  test "greets the world" do
-    assert ProxmoxAgent.hello() == :world
+  test "version/0 returns a non-empty string" do
+    assert is_binary(ProxmoxAgent.version())
+    assert ProxmoxAgent.version() != ""
   end
 end