fix: rewrite validator system prompt for clarity and correctness

[linbo328]

Summary

Fixes #2

The _SYSTEM_PROMPT in nodes/validate_node.py contained several issues:

Problems Fixed

1. Typo: "rsult" instead of "result"
2. Contradictory instructions: Multiple conflicting retry rules that confused the validator LLM
3. Dead code reference: Confidence threshold rule (< 0.95) referenced a value not available in the validation node state
4. Unprofessional grammar: Broken syntax and run-on sentences in the prompt

Changes Made

• Removed the dead confidence-threshold rule
• Removed redundant and contradictory retry instructions
• Rewrote the system prompt as a single, coherent validation rubric
• Fixed all grammar and spelling issues
• Kept the same validation criteria (factual correctness, relevance, detail, no hallucinations)

Testing

• The prompt now provides clear, non-contradictory instructions to the validator LLM
• All four original validation criteria are preserved
• The retry logic remains controlled by the graph structure, not the prompt

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