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/dev.exs

@@ -25,7 +25,7 @@ config :arcadia_cloud, ArcadiaCloudWeb.Endpoint,
 # Guardian — must match arcadia-core's Arcadia.Guardian dev secret_key
 # (shared verbatim with arcadia-social and arcadia-voice).
 config :arcadia_cloud, ArcadiaCloud.Guardian,
-  secret_key: "DuMkIRN3Qcxk8VqOu8nHj5i7a7a7YgBHF4oXqKwDI4A="
+  secret_key: "dev-only-guardian-secret-not-for-production-aaaaaaaaaaaaaaaa="
 
 # skyai-finance push — service-to-service identity for cloud invoice push.
 # tenant_id="platform-admin" lands invoices in the platform's own books;