chore(security): purge burned Guardian literal from dev/test config (Phase 4)

Replace the compromised committed Guardian secret with an obviously-fake
dev-only value. Prod reads GUARDIAN_SECRET_KEY from env and raises (Phase 1,
already merged); this removes the dead literal from the repo entirely.
Ref: arcadia-core/docs/GUARDIAN_SECRET_ROTATION.md Phase 4.

config/runtime.exs

@@ -32,7 +32,7 @@ config :arcadia_cloud, ArcadiaCloud.Guardian,
     System.get_env("GUARDIAN_SECRET_KEY") ||
       if(config_env() == :prod,
         do: raise("environment variable GUARDIAN_SECRET_KEY is missing"),
-        else: "DuMkIRN3Qcxk8VqOu8nHj5i7a7a7YgBHF4oXqKwDI4A="
+        else: "dev-only-guardian-secret-not-for-production-aaaaaaaaaaaaaaaa="
       )
 
 if config_env() == :prod do