chore: load amazing_print only in development and test

[mroderick]

amazing_print was declared in the default Gemfile group, so Bundler.require loaded it on boot in every environment, including production dynos. Nothing in the app uses it there: production logs go through CanonicalJsonFormatter, and the only consumer of the gem anywhere is SemanticLogger::Formatters::Color, which soft-requires it to pretty-print payload hashes in coloured text logs. That formatter is only relevant to development and test console output.

• Move the gem into group :development, :test, alongside the other dev-only gems
• Update the stale comment left over from the original rails_semantic_logger suggestion
• No Gemfile.lock change — the lockfile does not record groups

Review notes

• Single-line Gemfile change; the only behaviour change is that amazing_print is no longer loaded in production
• Deliberately not done: removing the gem entirely, which would lose coloured payload pretty-printing in the dev console