Clarify ripper-variant test gating: env-gated locally, CI-only by default

Previous wording implied both parser backends ran on every test
invocation. In reality the RipperRuby test class is gated by
'end if ENV["RDOC_USE_RIPPER_PARSER"]' at prism_ruby_test.rb:2503;
local bundle exec rake exercises only prism. CI runs the ripper variant
in a dedicated job. Note the env-var invocation an agent should use
locally before claiming a parser change is verified.

Author: claude

AGENTS.md

@@ -239,7 +239,7 @@ exe/
 - **Parsers:** Prism-based Ruby (default, `RDoc::Parser::PrismRuby`), legacy ripper-based Ruby (`RDoc::Parser::RipperRuby`, opt-in via `RDOC_USE_RIPPER_PARSER=1`), C, Markdown, RD
 - **Generators:** HTML/Aliki (default), HTML/Darkfish (deprecated), RI, POT (gettext), JSON, Markup
 
-Both Ruby parsers must produce equivalent code-object trees. Parser tests are written once in the `RDocParserPrismTestCases` module (`test/rdoc/parser/prism_ruby_test.rb`); both `RDocParserPrismRubyTest` and `RDocParserRipperRubyWithPrismRubyTestCasesTest` include it, so each test runs against both backends. Add new parser tests there rather than to a parser-specific class.
+Both Ruby parsers must produce equivalent code-object trees. Parser tests are written once in the `RDocParserPrismTestCases` module (`test/rdoc/parser/prism_ruby_test.rb`) and included by `RDocParserPrismRubyTest` and `RDocParserRipperRubyWithPrismRubyTestCasesTest`. The ripper variant is gated on `RDOC_USE_RIPPER_PARSER` (so a local `bundle exec rake` exercises only prism); CI runs the ripper variant in a separate job (`.github/workflows/test.yml`). Add new parser tests to the mixin so both backends pick them up, and run the ripper variant locally with `RDOC_USE_RIPPER_PARSER=1 bundle exec rake` before declaring a parser change done.
 
 ### Code Object Model and Constant Aliases