Stop injecting rails_12factor from the Heroku generator

[tricknotes]

Why

rails generate ember:heroku added rails_12factor to the project's Gemfile for the staging and production groups.

The gem exists to backport twelve-factor behaviour — logging to stdout and serving static assets in production — onto Rails 4, which Rails has done natively since 5.0. Its own README says a new Rails 5 application does not need it, and its repository was archived in February 2022.

Every Rails version this gem supports is >= 5.2 (README.md, "Ruby and Rails support"; the CI matrix runs 7.2, 8.0, 8.1 and main), so the injection could only ever add an unmaintained dependency to a project that has no use for it.

What changed

• Removed HerokuGenerator#inject_12factor_gem.
• Removed the "This will install the following gems: rails_12factor" line from the generator's USAGE.
• Added a spec asserting the generator leaves the Gemfile untouched. Renamed the create_empty_gemfile_for_bundler helper to create_empty_gemfile — with no gem action left, the file is now the fixture the assertion reads rather than a prerequisite for Thor.
• CHANGELOG entry with the upgrade note (remove rails_12factor from your Gemfile).

Verification

$ bin/rspec spec/generators
6 examples, 0 failures

The new spec was confirmed meaningful: with inject_12factor_gem restored it fails with +gem "rails_12factor", group: [:staging, :production] in the diff.

Full spec/lib also passes here except EmberCli::App#test exits with exit status of 0, which fails on main in this container too — ember test reports Launcher Chrome not found, unrelated to this change.

Note on merge order

This branch and #671 both edit lib/generators/ember/heroku/USAGE; whichever merges second needs a small conflict resolution.

🤖 Generated with Claude Code

https://claude.ai/code/session_014rAfkAVGVifbaEoTVeT66u

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