fix: stale bot message says 90 days but threshold is 60 days

## Description

`script/github/close-issues.ts` uses `const days = 60` but the bot comment says "90 days". The message should match the actual behavior.

## What needs to be fixed

Line 6: change "90 days" to "60 days" in the closure message to match the actual threshold on line 4.

## OpenCode version

dev branch
