refactor(loader): A3 — unificar loader, eliminar duplicación

El loader vivía partido: arje-brain/loader.rs cargaba EntityCards Y
Rules, mientras brahman-cards tenía su propia infra de card-loading.

Resolución por linaje:
- Card-loading (load_card_file, extract_card_from_json) → brahman-cards
  (entity_loader.rs). Toda card-loading del ecosistema vive ahí.
- Rule-loading (load_rules_file, extract_rules_from_json) → arje-brain-rules
  (loader.rs), junto a la definición de Rule.
- arje-brain/loader.rs eliminado.

arje-brain re-exporta ambos para compat de consumidores (arje-zero).
cargo check --workspace verde. Tests: 13 arje-brain-rules + 31 brahman-cards.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/runtime/arje-brain/src/introspect.rs

@@ -424,7 +424,7 @@ impl IntrospectServer {
                         "ReloadRules sin path y sin rules_out configurado".into()
                     ),
                 };
-                let rules = match crate::loader::load_rules_file(&path) {
+                let rules = match arje_brain_rules::load_rules_file(&path) {
                     Ok(r) => r,
                     Err(e) => return IntrospectResponse::Error(format!("load: {e}")),
                 };