Improve Rails compatibility CI messaging for experimental failures

[kitcommerce]

Closes #862.

Summary

• Adds clearer, human-readable messaging when an experimental Rails compatibility matrix entry fails during bundle install.
• Includes a link to the relevant tracking issue when known (currently WA-FWD: Upgrade Mongoid to support Rails 7.1/7.2 appraisals #841).
• Adds a short README note describing how to interpret the Rails compatibility job output.

Client impact

None. This is CI-only messaging/documentation; runtime behavior and release artifacts are unchanged.

Testing

• Not run locally (GitHub Actions workflow change).
• Validated workflow YAML parses cleanly.

[kitcommerce]

CI is currently failing early with:

The gemspecs for path gems changed, but the lockfile can't be updated because frozen mode is set

Fix: run bundle install and commit the resulting Gemfile.lock update (it should refresh the PATH gemspec checksums) so the CI jobs can install gems in frozen mode.

Once that’s pushed, we can proceed with reviews/merge.