E2E-4: Migration script idempotency (N4)

[Cho-Geer]

E2E-4: Migration script idempotency

Parent Epic: #87
Source: read-audit-db-migration-plan.md N4

Verify migrate-read-audit.ts is idempotent.

Commands

cd /home/zhaoge/workspace/opencode/work-one
# Run migration twice
bun .opencode/scripts/migrate-read-audit.ts 2>&1 | tail -5
bun .opencode/scripts/migrate-read-audit.ts 2>&1 | tail -5
# Second run should show 0 imported (skipped=N)