docs: reformat bilingual documents into separate blocks ...

- Reorganized Issue-2 journal, Issue Management rules, and Development Guide.
- Separated Japanese and English descriptions into distinct blocks for better readability.
- Issue-2.md (local sync file) was also updated but is excluded from git track.

feat: normalize database schema for document-chunk separation (Issue-2) ...

- Added 'documents' table for metadata.
- Refactored 'items' table for chunk storage.
- Updated MCP tools (add_item_text, search_text, etc.) to support the new schema.
- Updated technical documentation.