Add dcy-005 Discovery (draft): portfolio-hub asset snapshot read API

- new-discovery dcy-005: read-direction follow-up of dcy-004 (deferred
  get_assets/get_portfolios). trex-core exposes portfolio-hub asset snapshot
  as a read API, returning buy candidates with slot-occupied symbols filtered
  out (slot overlay) and an empty list when no slots remain.
- 6 open questions pinned: snapshot shape, freshness/cache, core-proxy policy,
  in-flight overlay (ledger outbox eventual consistency), availability degrade,
  empty-list semantics.
- Link dcy-004 후속 Discovery 참조 → dcy-005; add dcy-005 to discovery index.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/discovery/index.md

@@ -9,4 +9,5 @@
 | dcy-001_20260603_slot-pure-domain-migration | 20260603 | trex-core 베이스 수립 및 slot 순수 도메인 이식 | [Discovery](./dcy-001_20260603_slot-pure-domain-migration.md) | confirmed |
 | dcy-002_20260603_slot-core-daemon-http-api | 20260603 | trex-core 슬롯 코어 데몬화 + HTTP intent API | [Discovery](./dcy-002_20260603_slot-core-daemon-http-api.md) | confirmed |
 | dcy-003_20260604_broker-execution-and-outbox | 20260604 | 브로커 단일 세션 + 주문 집행 + durable outbox (불변식 B) | [Discovery](./dcy-003_20260604_broker-execution-and-outbox.md) | confirmed |
-| dcy-004_20260604_portfolio-hub-ledger-integration | 20260604 | portfolio-hub ledger 연동 — 체결을 외부 원장에 기록 | [Discovery](./dcy-004_20260604_portfolio-hub-ledger-integration.md) | confirmed |
+| dcy-004_20260604_portfolio-hub-ledger-integration | 20260604 | portfolio-hub ledger 연동 — 체결을 외부 원장에 기록 | [Discovery](./dcy-004_20260604_portfolio-hub-ledger-integration.md) | confirmed |
+| dcy-005_20260604_portfolio-hub-asset-snapshot-api | 20260604 | portfolio-hub 종목 스냅샷 read API — 매수 전략 모듈에 슬롯-제외 후보 제공 | [Discovery](./dcy-005_20260604_portfolio-hub-asset-snapshot-api.md) | draft |