Feat: Adopt TypeScript Template Literals for Custom Evaluator Prompts

[christso]

Summary

Enable the use of native TypeScript template literals for defining custom evaluator prompts in code-based evaluators. This provides type safety, better developer experience, and performance benefits over string-based templating with placeholders.

Problem

Currently, LlmJudgeEvaluator relies on string templates with {{variable}} placeholders. This approach:

• Lacks type safety: No compile-time check if variables exist in the context.
• Has limited logic: Conditional logic or loops require complex template syntax or are impossible.
• Is error-prone: Typos in placeholders are only caught at runtime.

Solution

1. Introduce a PromptTemplate function type that accepts EvaluationContext and returns a string.
2. Update LlmJudgeEvaluator to accept this function type.
3. Enhance the evaluator loader to support loading .ts files directly. Users can simply export a prompt function from a TypeScript file, and AgentV will load and use it as the evaluator template.

Impact

• Core: LlmJudgeEvaluator, orchestrator.ts, and loader logic.
• DX: Developers can write custom evaluators as simple TypeScript functions without boilerplate.
• Dependencies: May require adding a library like jiti to support runtime TypeScript loading.
• Backward Compatibility: Existing string-based templates will continue to work.