Operationalize unused helper methods linter

First added in !139028 (merged), move scripts/lint/unused_helper_methods.rb to a state where it can be run automatically on git push (via LeftHook).

Work remaining to be done:

• Improvements to unused helper methods linter (#436399 - closed) (weighted separately from this issue)
• Address "false positives` of routing helpers and class methods
• add to automatic linters run via LeftHook