fix(core): suppress noisy warning for inline llm-grader prompts

[christso]

Summary

• Added looksLikeFilePath() heuristic to detect inline prompts vs file paths
• Skips file resolution entirely for strings that are clearly inline prompts (multi-line, >200 chars, 3+ words)
• Only warns when string looks like a file path but wasn't found

Test plan

• 8 unit tests for looksLikeFilePath() covering file extensions, relative/absolute paths, multi-line, long strings, natural language, empty strings
• All 1731 tests pass
• Build, typecheck, lint clean

Closes #901

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