[Story]: Chargeback/showback export by cost centre

## Summary
Build a chargeback/showback export by cost centre, in a format finance tooling can consume, mapping per-developer AI spend (not headcount-divided estimates) to the customer's cost-centre structure.

## Intent / Source of truth
Buyers explicitly want per-developer attribution, not a flat per-headcount split — this is the named differentiator in FinOps buyer research. Part of [Epic] Per-BU cost attribution and chargeback reporting.

## Scope
- [ ] CSV export by cost centre / BU / team, per billing period
- [ ] API endpoint for programmatic export (finance-tool integration)
- [ ] Showback (read-only view) vs chargeback (exportable-for-billing) distinction

## Out of scope
- Direct integration with specific finance tools (SAP, NetSuite, etc.) — the export format is the deliverable, not bespoke connectors

## Verification
A generated export reconciles against the BU-rollup story's totals; a sample import into a common finance-tool CSV format (e.g. NetSuite journal import) validated manually.

## Risk assessment
An export that doesn't reconcile with the underlying ledger would produce incorrect customer invoices/chargebacks if used downstream for billing — needs the same reconciliation discipline as a5s1.

## AI Usage Declaration
Drafted with AI assistance during the 2026-08-13 cross-repo backlog consolidation and enterprise-readiness research. A human owns intent, verification and consequences.

Start by locating the existing BU-rollup story and the underlying ledger, then trace how cost-centre, BU, team, and per-developer attribution are represented. Done means a per-billing-period export and API endpoint distinguish showback from chargeback, reconcile with BU-rollup totals, and pass a manual sample import against a common finance-tool CSV format.

由索引模型根据 Issue 内容生成。