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.

docs: relocate rules to .agent/rules and make them bilingual ...

- Moved issue management and development guide to .agent/rules.
- Added English translations to all operational rules.
- Cleaned up obsolete files in docs/.