feat: Enhanced default system prompts

[ajianaz]

Summary

Improve default REVIEW_SYSTEM_PROMPT and SCAN_SYSTEM_PROMPT with anti-hallucination constraints, clearer JSON format instructions, and severity definitions. Referenced from old cora TS repo patterns.

Accept Criteria

• Add anti-hallucination constraints (ONLY comment on files in diff)
• Add explicit JSON format example in prompt
• Add clear severity level definitions with examples
• Add 'Return ONLY valid JSON' emphasis
• All existing tests still pass (prompt change is backwards compatible)
• No breaking change to parse logic

Scope

llm.rs constants only