ara check: per-rule config via .ara-check.toml

[EYH0602]

Follow-up to #39 (the ara check linter/format-checker).

For v1, ara check's fixable rule set (ARA001–ARA004) is hardcoded.
This issue tracks adding an optional project-level config so authors can
enable/disable rules and tune behavior, in the spirit of ruff's
pyproject.toml/ruff.toml.

Scope

• Discover a .ara-check.toml at the ARA dir (or repo root) if present.
• Per-rule enable/disable (e.g. select / ignore by rule id).
• Optionally: severity overrides and a global fixable/unfixable split.
• Sensible defaults so no config file is required (v1 behavior stays the default).

Notes

• Keep it self-contained (no external tool dependency), consistent with the
  ara check design in plans/ara-check-lint-fix-ci.md / docs/.