feat(holdings): REQ-043 현재 보유 평단(WAC) read — GET /v1/portfolios/{id}/holdings/cost-basis (BAT-018, PR #14)

dcy-008(confirmed)/REQ-043(Approved)/BAT-018. trex-core dcy-034 FR-7 cold-start seed 의존. AC 11/11, QC PASS. worktree+PR 워크플로 첫 파일럿.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VHtM5Ws1HKCKQ4JGi6GRb5

docs/batches/index.md

@@ -25,3 +25,4 @@
 | BAT-015 | standard | released | REQ-033, REQ-034, REQ-035 | dcy-005 | [bat-015](./bat-015_20260601_rel-001-feedback-cleanup/) |
 | BAT-016 | standard | released | REQ-036, REQ-037, REQ-038 | dcy-006 | [bat-016](./bat-016_20260605_ledger-accounting-consolidation/) |
 | BAT-017 | standard | released | REQ-039, REQ-040, REQ-041, REQ-042 | dcy-007 | [bat-017](./bat-017_20260618_trade-history-and-pnl-api/) |
+| BAT-018 | batch-lite | in-progress | REQ-043 | dcy-008 | [bat-018](./bat-018_20260625_holdings-cost-basis-endpoint/) |