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>

app/lib/arcadia-knowledge.ts

@@ -1,5 +1,5 @@
 // Domain primer baked into the assistant's system prompt so it understands
-// what arcadia-app is, what platform admins do, and how the data model fits
+// what arcadia-core is, what platform admins do, and how the data model fits
 // together. Keep this tight — it costs context tokens on every turn.
 
 export const ARCADIA_KNOWLEDGE = `Arcadia (the backend you administer):
@@ -31,7 +31,7 @@ Things to keep in mind when assisting:
 - Writes are auditable. Suggest the user double-check tenant slug and impact before suspend/deactivate. Deactivate is harsher than suspend — only use when clearly intended.
 - The operator can impersonate tenant users for debugging (POST /api/v1/admin/impersonate/:user_id) — surface this when they ask "why can't user X log in".
 - Quotas / rate cards / billing config errors usually surface as 402/403 from /api/v1 endpoints — diagnose by checking the tenant's billing-config and api-metering quotas.
-- The reference Phoenix app lives at \`reference/arcadia-app/\` in the workspace; its OpenAPI spec is at /api/openapi (sync via \`node ../lib-arcadia-core-client/scripts/sync-spec.mjs\`).
+- The reference Phoenix app lives at \`reference/arcadia-core/\` in the workspace; its OpenAPI spec is at /api/openapi (sync via \`node ../lib-arcadia-core-client/scripts/sync-spec.mjs\`).
 - Search admin (arcadia-search) is a separate service. Manage tenants/corpora at \`/search\`. Use \`list_search_corpora\` if you don't know what's indexed; \`rebuild_search_corpus\` after uploads or when results look stale; \`search_kb\` / \`read_chunk\` to query.
 
 When the user asks something that maps to a tool, call it. When they ask about a concept, explain it from this primer in plain language. Write tools (suspend_tenant, activate_tenant) prompt the operator with an inline confirm card before they actually run — you do not need to ask in prose first; just call the tool and the user will see the confirmation UI. If the user denies a write, do not retry it; ask what they'd like to do differently.