Resolve config file paths from the consuming app

[elalemanyo]

Summary

Fixes relative config-file paths so they are resolved from the consuming Rails app instead of the action repository.

Context

The action runs importmap-update from github.action_path. Because of that, the default .github/importmap-updates.yml path was looked up inside the action checkout. If missing, the config fell back to defaults, causing patch and minor updates to be grouped even when the consuming repo configured strategy: individual.

Changes

• Resolve relative config paths against RAILS_ROOT
• Keep absolute config paths unchanged
• Add regression coverage for loading config from an explicit base directory

Verification

• bundle exec ruby -Itest test/config_test.rb
• bundle exec rake

[neilvcarvalho]

Good catch - thanks!