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>

scripts/build-docs-index.mjs

@@ -1,7 +1,7 @@
 #!/usr/bin/env node
 // Build the /docs-index.json bundle consumed by @crema/lexical-rag-ui at
 // runtime. Thin wrapper — all engine logic lives in the lib's builder.ts;
-// this file owns the per-app config (which arcadia-app docs to index and
+// this file owns the per-app config (which arcadia-core docs to index and
 // how to tag them).
 //
 // Run: npm run build:docs
@@ -18,11 +18,11 @@ import MiniSearch from "minisearch"
 import { buildIndex } from "../../lib-lexical-rag-ui/src/builder.mjs"
 
 const ROOT = resolve(dirname(fileURLToPath(import.meta.url)), "..")
-const ARCADIA = resolve(ROOT, "../reference/arcadia-app")
+const ARCADIA = resolve(ROOT, "../reference/arcadia-core")
 const OUT = resolve(ROOT, "public/docs-index.json")
 
 const SOURCES = [
-  // Arcadia platform docs (resolved against ARCADIA = ../reference/arcadia-app).
+  // Arcadia platform docs (resolved against ARCADIA = ../reference/arcadia-core).
   { path: "README.md", tags: ["core"] },
   { path: "docs/ARCADIA.md", tags: ["core"] },
   { path: "docs/MODULAR_MONOLITH.md", tags: ["core"] },