Skill: apt-repo-key-rotation

Purpose

When a vendor rotates its APT signing key, apt update logs an Err for that one repo and quietly keeps using the stale index, so upgrades from it stop arriving. The fix is to replace the keyring the repo's signed-by points at with the vendor's current key. Don't use apt-key, which is gone from current Debian and Ubuntu releases.

Steps

1. Find the keyring apt actually uses

Don't guess the path. Vendor docs say /usr/share/keyrings/..., but older installs often use /etc/apt/keyrings/..., and writing to the wrong one changes nothing.