chore: remove obsolete skill and cursor rule files

Remove openapi-architecture skill and biome-standards cursor rule
as they are no longer used in the current project structure.

- Remove .claude/skills/openapi-architecture/SKILL.md
- Remove .cursor/rules/biome-standards.mdc

These files contained outdated references to bun and are not needed
after migrating to the pnpm-based workflow.

Author: ryoppippi