feat(intents): dcy-011 buy intent portfolio_id 필수 계약 + ledger skip 가시화 (BAT-015)

dcy-011 전 주기 delivery. issue #6(buy 식별자 정정) + issue #11(ledger silent skip) 흡수.
거버넌스: 기존 Implemented REQ는 change-req, 신규 기능만 신규 REQ(OQ-3).

- REQ-007 (change-req): buy admission portfolio_id 필수화 + SnapshotStore symbol
  resolve + unknown_portfolio 거부 (hard cutover, OQ-1/OQ-2)
- REQ-021 (change-req): buy 메시지 payload symbol 제거
- REQ-018 (change-req): _record_ledger skip 가시화 (WARN + ledger_skipped 카운터)
- REQ-055 (신규): closed↔ledger_outbox anti-join 운영 점검 find_ledger_gap_slots

코드: intents.py(BuyIntent portfolio_id 필수 + resolve_symbol 주입 + _admit_buy
reject 분기), messaging_handlers.py, runtime/daemon.py(_resolve_symbol 배선),
execution.py(skip WARN 로그), adapters/persistence/ops_checks.py(신규 anti-join).

검증: 단위 282 passed/32 skipped, 인프라 게이트(PG+RabbitMQ) 314 passed, ruff clean.
BAT-015(standard) release-candidate. srs/index Next ID → REQ-056.

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

docs/batches/index.md

@@ -22,3 +22,4 @@
 | BAT-012 | standard | release-candidate | REQ-043, REQ-044, REQ-045, REQ-046, REQ-047 | dcy-008 | [bat-012](./bat-012_20260607_kill-switch-control/) |
 | BAT-013 | standard | release-candidate | REQ-048, REQ-049, REQ-050, REQ-051 | dcy-009 | [bat-013](./bat-013_20260607_multi-instance-topology/) |
 | BAT-014 | batch-lite | release-candidate | REQ-052, REQ-053, REQ-054 | dcy-010 | [bat-014](./bat-014_20260608_safety-sell-cooldown/) |
+| BAT-015 | standard | release-candidate | REQ-007 (CHG-20260608-01), REQ-018 (CHG-20260608-01), REQ-021 (CHG-20260608-01), REQ-055 | dcy-011 | [bat-015](./bat-015_20260608_buy-intent-portfolio-id-contract/) |