Correct a Ramp bill line without touching the wrong bill

**Objective:** On bill `{bill-id}`, change the line whose line number or memo matches `{line-reference}`, setting amount `{new-amount}` (leave empty to keep it) or memo text from the user's request.

**Step 1: Read the bill**

Call `ramp-get-bill` with `id` set to the bill id and `ramp-get-bill-line-item` with `bill-vendor-id` from the bill. Keep only lines whose bill invoice number matches. If the line reference matches none or several, list the lines and stop.

**Step 2: Confirm**

Show the line as it is and as it will be. Warn if the bill's approval status is already approved or its payment status is not unpaid. Ask for explicit confirmation.

**Step 3: Change**

Call `ramp-bill-update-line-item` with the bill id, the line id and only the fields that change. Re-read the line and report the stored values.

**Rules & Guidelines:**

* Only `ramp-bill-update-line-item` may write, once, after confirmation.
* Never call `ramp-bill-delete-line-item`, `ramp-bill-archive` or any inventory line tool. Removing lines is not part of this job.
* Refuse to edit a bill that is paid.

**Validation:**

* If no new amount or memo was given, stop and ask what to change.