refactor: rename service references arcadia-app → arcadia-core

The Phoenix auth/identity/tenancy backend repo is being renamed
arcadia-app → arcadia-core (its primary OTP app is already arcadia_core).
Updates prose, doc paths, and git.sky-ai.com repo URLs. Deliberately
leaves the Rust crate arcadia-app-client and host arcadia-app.internal
(handled separately), and the kept namespace (issuer/release "arcadia").

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/arcadia_cloud/guardian.ex

@@ -2,11 +2,11 @@ defmodule ArcadiaCloud.Guardian do
   @moduledoc """
   Verify-only Guardian implementation.
 
-  arcadia-cloud never issues tokens — that is arcadia-app's job. We only
+  arcadia-cloud never issues tokens — that is arcadia-core's job. We only
   decode and verify tokens minted by `Arcadia.Guardian`, then expose the
   claims as a lightweight identity struct.
 
-  Token contract (set by arcadia-app):
+  Token contract (set by arcadia-core):
     sub          => "<user_id>:<tenant_id>"
     tenant_id    => UUID string
     tenant_slug  => string
@@ -36,7 +36,7 @@ defmodule ArcadiaCloud.Guardian do
 
   @doc """
   Dev/test helper: mint a JWT using the local secret. Production tokens
-  are minted by arcadia-app.
+  are minted by arcadia-core.
   """
   def mint_dev_token(claims_overrides \\ %{}) do
     defaults = %{

lib/arcadia_cloud_web/plugs/require_auth.ex

@@ -1,6 +1,6 @@
 defmodule ArcadiaCloudWeb.Plugs.RequireAuth do
   @moduledoc """
-  Validates a Bearer JWT issued by arcadia-app and assigns the resulting
+  Validates a Bearer JWT issued by arcadia-core and assigns the resulting
   identity + raw claims onto the conn. Halts with 401 on any failure.
 
   Downstream controllers read `conn.assigns.current_identity` and, if