Record a Rillet bill payment without overpaying

**Objective:** Record a payment against the vendor bill with expense number `{expense-number}`, for `{amount}` (default the full unpaid balance), dated `{payment-date}` (default today), from the cash account `{account-code}` (ask if not given).

**Step 1: Find the bill**

Call `rillet-get-bills` filtered by `expense-number` = `{expense-number}`. If none or more than one match, list them with vendor and bill date and stop. Call `rillet-get-vendors` to name the vendor.

**Step 2: Work out the balance**

* Bills carry no total. Call `rillet-get-bill-lines` filtered by `expense-number` and sum `amount` plus `tax-amount`; take the `currency` from the lines.
* Call `rillet-get-bill-payments` filtered by `bill-id` and sum `amount`.
* Unpaid = total minus payments. If `{amount}` is empty, use the unpaid balance. If `{amount}` is more than the unpaid balance, stop and say so.

**Step 3: Confirm the cash account**

If `{account-code}` is empty, call `rillet-get-accounts` and offer the active asset accounts that look like bank or cash accounts. Either way, confirm the account exists and is active.

**Step 4: Confirm, then write once**

Show the user: vendor, expense number, bill date, due date, total, already paid, this payment, date, currency, account, and what remains after. Ask them to confirm. Only after they say yes, call `rillet-add-bill-payment` once, with `bill-id`, `amount`, `currency`, `date` and `account-code`.

**Step 5: Verify**

Call `rillet-get-bill-payments` filtered by `bill-id` and show the new payment and the remaining balance.

**Rules & Guidelines:**

* `rillet-add-bill-payment` is the only write tool you may call, and only once, after confirmation.
* Never call `rillet-update-bill`; it replaces the whole bill and clears anything left empty.
* If the call fails, report the error and do not retry with changed values unless the user asks.

**Validation:**

* If the bill is already fully paid, say so and stop.
* If `{amount}` is given but not a positive number, ask again.