Random pick never returns the last issue

## Bug
In bin/good-first-issue.js the issue selection uses:

const key = cmd.first ? 0 : Math.floor(Math.random() * Math.floor(issues.length - 1))

Because the multiplier is bounded by \`issues.length - 1\`, \`Math.random()\` can never produce \`issues.length - 1\`, so the final issue in the result list is never selectable.

## Expected
Every returned issue should have an equal chance of being picked, including the last one.

## Fix (one line)
const key = cmd.first ? 0 : Math.floor(Math.random() * issues.length)

The inner \`Math.floor(issues.length - 1)\` is also redundant since \`issues.length\` is already an integer.

Open bin/good-first-issue.js and inspect the issue-selection expression described in the report. Verify the random selection behavior with a result list that includes a final issue; done means every issue, including the last one, can be selected with equal probability.

Written by the indexing model from the issue text.