feat(renovate): group Chezmoi updates into a single PR

[Copilot]

Renovate was opening separate PRs for home/.chezmoiversion and .mise.toml when Chezmoi released a new version. Because each PR only updated one file, the other remained stale — causing CI to fail on version mismatch checks until both PRs merged.

Changes

• renovate.json5 — adds a packageRules entry matching twpayne/chezmoi with groupName: "chezmoi", causing Renovate to bundle both the custom-manager .chezmoiversion update and the inline-comment .mise.toml update into one atomic PR:

  packageRules: [
    {
      groupName: "chezmoi",
      matchDepNames: ["twpayne/chezmoi"],
    },
  ],

• tests/bash/install-chezmoi-version.bats — adds a test asserting the packageRules/groupName/matchDepNames grouping is present in the Renovate config.