* refactor(plugins): move BaseAgenticAnswerStep into lme/beam plugins
Move the shared benchmark answer base class from the core
reme.steps.benchmark package into each plugin's own src tree
(reme_lme.base_agentic_answer / reme_beam.base_agentic_answer) with
absolute imports, drop the core benchmark steps package, and update
plugin READMEs accordingly.
* refactor(plugins): simplify benchmark answer steps
* docs(plugins): clarify benchmark answer step migration
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Co-authored-by: jinli.yl <jinli.yl@alibaba-inc.com>