docs: reformat bilingual documents into separate blocks
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- 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.
docs: renumber specifications to ensure continuous sequence
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- Renamed 06_development_guide.md to 05_development_guide.md.
- Renamed 07_ui_design_spec.md to 06_ui_design_spec.md.
- Updated journal to reflect the changes.
