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

.gitignore

@@ -43,4 +43,6 @@ htmlcov/
 
 # OS junk
 .DS_Store
-Thumbs.db
+Thumbs.db
+# Agent worktree 격리 디렉토리 (커밋 금지)
+.claude/worktrees/