docs: add Evaluate section to README

[njbrake]

Surface the LlmJudge and AgentJudge evaluators with short snippets and a pointer to docs/evaluation.md, so users hitting the README see that evaluation is built in. Closes a documentation gap that pushed users to look elsewhere for grading agent runs.

Description

PR Type

• New Feature
• Bug Fix
• Refactor
• Documentation
• Infrastructure

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• I understand the code I am submitting.
• I have added unit tests that prove my fix/feature works.
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• Documentation was updated where necessary.
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