refactor(core): centralize prompt discovery in PromptLoader and cleanup AssertionEngine

[silkiy]

Description

This PR addresses structural issues and dead code identified in AssertionEngine.ts (Issue #43). The logic for resolving prompt file paths has been extracted from the engine and centralized into a dedicated service within PromptLoader.

Key Changes

• Cleaned up AssertionEngine: Removed the unused searchPaths variable and deleted the legacy loadPrompt method. The engine now delegates path resolution to PromptLoader.
• Implemented PromptLoader.resolveAndLoad: Centralized the discovery logic with the following priority:
  1. User Config: Respects promptsDir set in evaliphy.config.ts.
  2. SDK Dist: Fallback to bundled prompts in ./prompts.
  3. SDK Source: Fallback to source prompts in ../../prompts (useful for development).
• Robust Error Handling: Improved the FILE_NOT_FOUND and PROMPT_LOAD_ERROR handling to provide clearer debugging information to users.
• Comprehensive Unit Tests: Added promptLoader.test.ts covering all resolution scenarios, fallback mechanisms, and frontmatter validation using vitest.

Validation

• Ran pnpm exec tsc --noEmit (Type-check passed).
• Ran pnpm exec vitest run packages/assertions/tests/judge/promptLoader.test.ts (All tests passed).

Closes #43

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[priyanshus]

Can we break this function further?

[silkiy]

Good catch! I agree the previous function was doing too much. I've broken it down into smaller, single-responsibility private methods (checkUserConfig, checkSdkDist, and checkSdkSource) so resolvePromptPath just acts as a clean orchestrator now. Just pushed the update!

[priyanshus]

@silkiy Please check the tests. A few failed. Thanks.

[priyanshus]

Tests are failing.