Add '--list-checks' flag that lists all implemented checks

[vickgoodman]

Usage example:

$ beman-tidy --list-checks
[Requirement]    cmake.library_alias
[Recommendation] cmake.library_name
[Recommendation] cmake.project_name
[Recommendation] cpp.extension_identifiers
[Recommendation] cpp.namespace
[Requirement]    directory.docs
[Requirement]    directory.examples
[Requirement]    directory.papers
[Requirement]    directory.sources
[Requirement]    directory.tests
[Recommendation] file.copyright
[Requirement]    file.license_id
[Recommendation] file.names
[Requirement]    file.test_names
[Recommendation] library.name
[Recommendation] license.apache_llvm
[Requirement]    license.approved
[Requirement]    license.criteria
[Requirement]    readme.badges
[Recommendation] readme.implements
[Requirement]    readme.library_status
[Requirement]    readme.license
[Recommendation] readme.purpose
[Recommendation] readme.title
[Requirement]    release.github
[Recommendation] release.godbolt_trunk_version
[Recommendation] release.notes
[Requirement]    repository.code_review_rules
[Requirement]    repository.codeowners
[Requirement]    repository.default_branch
[Recommendation] repository.disallow_git_submodules
[Requirement]    repository.name
[Requirement]    toplevel.cmake
[Requirement]    toplevel.license
[Requirement]    toplevel.readme

$ beman-tidy --list-checks /path/to/repo outputs the same

[Copilot]

Pull request overview

This PR adds a new --list-checks CLI flag intended to print all implemented Beman Standard checks (with their Requirement/Recommendation type), and updates the README’s captured --help output accordingly.

Changes:

• Add --list-checks handling to the CLI, printing registered check names and their types.
• Update README --help snippet to include --list-checks (and reflect current CLI options).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
beman_tidy/cli.py	Adds --list-checks via a custom argparse action and implements check listing output.
README.md	Updates the documented --help output to include the new flag.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[neatudarius]

Works and mirrors the --version action pattern. Two inline notes below. Also worth a CLI smoke test for --list-checks (asserts exit 0 + non-empty list) — there's no CLI test coverage today.

[neatudarius]

This works only because the check modules' @register_... decorators run as a side effect of the from ...pipeline import run_checks_pipeline import above. If that import ever becomes lazy, --list-checks silently prints an empty list. Suggest importing the check modules explicitly here (or a comment noting the dependency).

[neatudarius]

Duplicates the config-load guard in main(), and the exit codes differ: here exit 1, but main()'s early return exits 0. A shared helper would keep them consistent.