fix(purge): restore cursor when purge exits early

[YuanHanzhong]

Summary

• Restore the cursor from a cleanup trap after mo purge hides it.
• Preserve the purge command exit status while still running cleanup.
• Add a focused regression test for early purge exit after cursor hide.

Testing

• bash -n bin/purge.sh tests/purge.bats
• shellcheck --rcfile .shellcheckrc bin/purge.sh tests/purge.bats
• BATS_FORMATTER=tap bats tests/purge.bats -f "mo purge restores cursor when purge exits after hiding it"
• git diff --check

Notes

• The full tests/purge.bats suite still has two unrelated tilde-expansion failures on upstream main.

Fixes #915

[tw93]

@YuanHanzhong Thanks for jumping on this cursor restore bug. I merged #928, which covers the same #915 issue and also keeps the existing temp-file cleanup trap intact across exit paths. Closing this one as a duplicate so we do not carry two competing implementations for the same fix.