Simplify describe.py vision analysis prompt and add focus guidance
- Replace describe_json.txt prompt with simplified 4-field format:
visual_description, primary (string), secondary (string), overlap (bool)
- Switch initial categorization from GEMINI_FLASH to GEMINI_LITE
- Remove unused box_2d cropping logic (_denormalize_box, _box_qualifies,
_crop_to_box methods and MIN_BOX_SIZE constant)
- Add focus guidance to follow-up prompts based on detected categories:
- Single category: no guidance
- Both same type: "Focus on both the X and Y content"
- Different types: "Focus on X content, not the Y content"
- Update screen.py formatter to read primary as string (with visible fallback)
- Update test fixtures to match new format
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>