fix: increase MAX_DIFF_SIZE default from 800KB to 5MB

Allows larger PRs to be reviewed without manual configuration.
The 800KB limit was too restrictive for many real-world PRs.

Author: LearningCircuit

ai-reviewer.sh

@@ -29,7 +29,7 @@ fi
 AI_MODEL="${AI_MODEL:-moonshotai/kimi-k2-thinking}"
 AI_TEMPERATURE="${AI_TEMPERATURE:-0.1}"
 AI_MAX_TOKENS="${AI_MAX_TOKENS:-64000}"
-MAX_DIFF_SIZE="${MAX_DIFF_SIZE:-800000}"  # 800KB default limit (~200K tokens, matching model context size)
+MAX_DIFF_SIZE="${MAX_DIFF_SIZE:-5000000}"  # 5MB default limit (allows large PRs while preventing excessive API usage)
 EXCLUDE_FILE_PATTERNS="${EXCLUDE_FILE_PATTERNS:-*.lock,*.min.js,*.min.css,package-lock.json,yarn.lock}"
 
 # Read diff content from stdin