#217 swamp-extension-model skill: include description field in upgrades quickref

The swamp-extension-model skill's "Version Upgrades" quickref shows the no-op upgrade pattern as:

upgrades: [
  {
    fromVersion: "...",
    toVersion: "...",
    upgradeAttributes: (old) => old,
  },
],

But the validator requires a description: string field on every upgrade entry. Authors who copy from the quickref hit a fairly opaque load-time error on first describe:

Failed to load user model "<name>.ts":
"Error: upgrades.0.description: Invalid input: expected string, received undefined"

Suggested fix: update the quickref's no-op example to include the description field, e.g.:

upgrades: [
  {
    fromVersion: "2026.01.01.1",
    toVersion: "2026.01.02.1",
    description: "No-op upgrade — globalArguments unchanged.",
    upgradeAttributes: (old) => old,
  },
],

references/upgrades.md may already document this, but the quickref is what most authors copy from.

Hit while authoring @4chems/gitlab-runner v2026.05.03.2 today.

(Supersedes #216 — that one had a mangled title from a shell-quoting accident on submission.)