Release v0.6.0

Breaking changes

• Many citation-related functions have been moved to the new
  citeme package (>= 0.1.1).
  The following functions are no longer part of checklist:
  citation_meta(), default_organisation(), get_available_organisations(),
  get_default_org_list(), new_org_list(), org_item, org_list,
  organisation, read_organisation(), store_authors(),
  upload_zenodo(), use_author(), write_citation_cff(),
  write_organisation(), and write_zenodo_json().
• bookdown_zenodo() is removed.
• add_badges() is renamed to add_checklist_badges().
• check_codemeta() is removed because it provided not much extra information.
  This also allows us to remove a dependency.
• create_draft_pr() and get_branches_tags() are removed to remove the
  dependency on gh.
  Note that we now have a GitHub template for pull requests.

New functions

• add_agents() installs AI agent configuration files (.github/agents/) in
  a package repository.
  create_package() and setup_package() now call this function automatically.
• add_issue_templates() adds GitHub issue and pull request templates to a
  package repository.
  create_package() and setup_package() now call this function automatically.
• fix_duplicate_git_config() removes duplicate entries from the local git
  config, fixing issues that could prevent clean_git() from working (#191).

Other changes

• check_package() gains a timeout argument to control the maximum time
  allowed for the CRAN checks.
• check_cran() now displays the full output of all failing tests.
• check_filename() allows Makevars, Cargo.toml, Cargo.lock, and
  *.agent.md file names.
• check_lintr() ignores the tests/testthat/_problems folder.
• check_spelling() improves the handling of Markdown URLs (#190) and world
  clock time-out errors.
• Submodule directories are ignored when running check_lintr(),
  check_filename(), check_folder(), and check_spelling() (#142).
• Updated compatibility with roxygen2 >= 8.0.0.
• Add the CRANhaven repository to reduce the
  impact of recently archived CRAN packages.
• clean_git() handles duplicate entries in git config.
• Reduce the number of layers in the Docker image.