Harden NoActor check with runtime environment guard

Add Mix.env() check to match?/3 for defense in depth.
Document NoActor pattern in CODE_GUIDELINES.md.

lib/mv/authorization/checks/no_actor.ex

@@ -58,7 +58,8 @@ defmodule Mv.Authorization.Checks.NoActor do
   @impl true
   def match?(nil, _context, _opts) do
     # Actor is nil
-    if @allow_no_actor_bypass do
+    # Double-check: compile-time AND runtime environment
+    if @allow_no_actor_bypass and Mix.env() == :test do
       # Test environment: Allow all operations
       true
     else