refactor(agents): Restructure Librarian prompt with clear request classification flow

- Reorganized prompt into Phase 0/1/2 workflow for systematic request handling
- Introduced 4 request types (TYPE A/B/C/D) for proper classification
- Removed ASCII art diagrams to simplify documentation
- Reduced prompt from 330 to 232 lines while maintaining clarity
- Improved flow between context gathering and decision-making phases

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Author: code-yeongyu