[Story]: Cost-centre mapping import from HRIS taxonomy

## Summary
Build a cost-centre mapping import so a customer can upload/sync their HRIS team taxonomy and have it drive the BU-rollup and chargeback structure, instead of manually re-entering it.

## Intent / Source of truth
Buyers expect chargeback mapped to their existing HRIS team taxonomy, not a lightbridge-specific structure they must maintain separately. Part of [Epic] Per-BU cost attribution and chargeback reporting.

## Scope
- [ ] CSV/API import of cost-centre-to-org-node mapping
- [ ] Validation and conflict reporting (unmapped users, ambiguous mappings)
- [ ] Re-sync workflow for periodic HRIS updates

## Out of scope
- Live HRIS system integrations (Workday, SAP SuccessFactors connectors) — file/API import is the scope; direct connectors are a future epic if demanded

## Verification
A sample HRIS export imports cleanly and correctly populates cost-centre mapping on the org tree; validation correctly flags an intentionally malformed sample file.

## Risk assessment
An import that silently drops or mis-maps users would corrupt chargeback accuracy; validation and conflict reporting must be strict, not best-effort.

## 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.

No files, tests, or entry points are named in the issue. First locate the existing org-tree, BU-rollup, and chargeback mapping entry points, then clarify the CSV/API contract and re-sync behavior. Done means a sample HRIS export imports cleanly, populates the cost-centre mapping, and a malformed sample produces strict validation conflicts.

Written by the indexing model from the issue text.