Vendor billing reconciliation

The vendor-billing tools answer a question that otherwise takes days of opening contracts: are we billing customers for everything the vendor charges us — and at the right price? Pax8 is the first vendor adapter.

The flow

1. Import vendor invoices (autotask_vendor_import_invoice, or a scheduled autotask_vendor_sync). Every line is kept with its charge class and cadence. Nothing in Autotask changes.
2. Map vendor customers and SKUs to Autotask companies and products; suggestions come with evidence.
3. Reconcile (autotask_reconcile_vendor_to_autotask): per customer and product — billed correctly, not billed, quantity differs, priced under cost, unlinked service — confirmed against posted billing items, not just contract prices.
4. Price with a policy (autotask_vendor_pricing_policy): e.g. vendor list price plus a percentage, with a margin floor when list is below cost.
5. Fix with change plans: price adjustments, missing contract lines, back-bills — each reviewed by hash before execution, with customer price-change notices recorded before customer-facing changes run.
6. Before invoicing, autotask_billing_preinvoice_check catches duplicates and gaps; exceptions get an outcome (absorbed, billed elsewhere, fix planned), an owner and a due date, and the monitor flags overdue ones.

Full details: docs/VENDOR_BILLING.md.