Plan the next payment run from Rillet bills due

**Objective:** List every vendor bill due in the next `{days-ahead}` days (default 14), plus anything already overdue, with the unpaid balance of each, optionally limited to the vendor `{vendor-name}`, so a payment run can be planned.

**Step 1: Resolve the vendor (only if one was given)**

If `{vendor-name}` is not empty, call `rillet-get-vendors` (it takes no filters) and find the vendor whose `name` matches. If more than one matches, list them with their `vendor-id` and stop. If none match, say so and stop.

**Step 2: Pull the bills**

Call `rillet-get-bills` with `due-date.to` set to today plus `{days-ahead}` days, and `vendor-id` if Step 1 ran. Do not set `due-date.from`, so overdue bills come back too.

**Step 3: Work out what is unpaid**

* Bills carry no total. For each bill, call `rillet-get-bill-lines` filtered by `expense-number` and sum `amount` plus `tax-amount` across its lines.
* Call `rillet-get-bill-payments` filtered by `bill-id` and sum `amount`.
* Unpaid = bill total minus payments. Drop bills with nothing unpaid.

**Step 4: Report**

* Overdue first, then by `due-date`: vendor name, expense number, bill date, due date, days until due (negative if overdue), total, paid, unpaid, currency.
* Totals of unpaid per currency, split into overdue and due-soon.
* Call `rillet-get-vendors` if you have not already, to put a name on every `vendor-id`.

**Rules & Guidelines:**

* This is read-only. Do not call any tool whose name starts with rillet-add-, rillet-update-, rillet-apply-, rillet-unapply-, rillet-record-, rillet-set- or rillet-setup-. In particular do not call `rillet-add-bill-payment`.
* Leave out bills whose `status` shows they are draft or void, and say how many.
* Never add amounts in different currencies together.

**Validation:**

* If nothing is due in the window, say so and give the next due date you can see, if any.