Track overdue supplier invoices

# Instructions — AP Aging: overdue invoices by bucket

## ⚠️ Critical rule on currencies

`spendesk_get_due_invoices` returns each invoice's amount in **its own spending currency** (e.g. GBP for a GBP invoice). **Do not sum these amounts together.**

The `totalAmount` field per bucket is in **functional currency** (EUR for a EUR company) — this is the one to use for totals.

Example observed on a real account:
- 1-30 day bucket: 500,000 GBP + 7,373,000 EUR → **INVALID** to sum
- Bucket `totalAmount`: 79,525.29 EUR → correct value

## Step 1 — Identify the company

Call `list_companies` for the company ID. The functional currency is then read from `summary.currency` in the step 2 response (`list_companies` does not return it).

## Step 2 — Retrieve the aging

Call `spendesk_get_due_invoices` with:
```json
{
  "companyId": "...",
  "asOfDate": "YYYY-MM-DD",
  "includeLineItems": true
}
```

Optional: add `"includeUpcoming": true` to also see invoices with a future due date.

## Step 3 — Present the results

**Use `totalAmount` per bucket** for totals, not the sum of individual amounts.

```
📊 Supplier outstanding balance as of <asOfDate>
Total overdue: summary.totalOverdueAmount EUR

Bucket          | Total (functional EUR) | Nb invoices | Max aging
1-30 days       | X EUR              | N           | 30d
31-60 days      | X EUR              | N           | 60d
61-90 days      | X EUR              | N           | 90d
90+ days        | X EUR              | N           | summary.oldestInvoiceDaysOverdue
```

For each invoice listed:
- Display the amount IN ITS OWN CURRENCY (do not convert manually) + indicate the currency
- Indicate: supplier, invoice number, due date, days overdue, cost center
- Flag invoices 90+ days as priority

## Note on scope

Only `invoicePurchase` with `paymentStatus=toPay` are included. Expense reports, card purchases, and credit notes are excluded. For a full view of all payables with a past dueDate, use `get_payables` with a `dueDate before today` filter.

## Example prompt

- "Which supplier invoices are overdue?"
- "Show the AP aging as of today"

⚠️ A past date in `asOfDate` does not rebuild a historical aging: the tool ages the invoices still unpaid today, at today's balances. Do not present it as the aging of a past close.