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>

docs/RAG.md

@@ -14,7 +14,7 @@ shape.
 |---|---|---|
 | Lib / service | `@crema/lexical-rag-ui` | `arcadia-search` (Rust) |
 | Engine | MiniSearch (BM25, JS) | Tantivy (BM25, Rust) |
-| Where it runs | In the user's browser | Sibling of arcadia-app |
+| Where it runs | In the user's browser | Sibling of arcadia-core |
 | Index storage | Static JSON, fetched once | mmap'd disk, ~30–80MB resident |
 | Practical corpus size | ~5–10MB / ~50–100k chunks | GB-scale, no hard cap |
 | Update cadence | Static — rebuilt at app build time | Live — cron, webhook, or admin trigger |
@@ -58,7 +58,7 @@ The index is a single JSON file built offline by
 **How it's wired here.**
 
 - Build script: `arcadia-admin/scripts/build-docs-index.mjs` reads
-  markdown from `../reference/arcadia-app/`, chunks at H1–H3,
+  markdown from `../reference/arcadia-core/`, chunks at H1–H3,
   produces `public/docs-index.json`. Runs on `npm run build:docs`
   (and as the `prebuild` step before `npm run build`).
 - Tool wrapper: `app/lib/admin-tools.ts` constructs a singleton
@@ -130,7 +130,7 @@ text extraction, webhook signature, service tokens) see
 
 The default deploy runs **both**:
 
-- `search_docs` indexes the same arcadia-app docs the parity corpus
+- `search_docs` indexes the same arcadia-core docs the parity corpus
   on `arcadia-search` indexes. Same content, two engines.
 - This is intentional — it means the assistant always has *something*
   to search, even if `arcadia-search` is down or unreachable. The