Description

Add structured skill files (.claude/skills/) to codify project conventions for all AI agents and contributors.

Context

Project conventions were previously scattered across AGENTS.md. This change modularizes them into individual skill files for better maintainability and enforceability across all agents (Claude, Codex, Cursor, etc.).

Scope

• Architecture skill: file/directory structure, process boundaries
• Commit skill: structured commit workflow with branch guard
• PR skill: pull request workflow with issue association
• Testing skill: test quality standards and workflow
• i18n skill: internationalization workflow (refactored)
• File structure conventions document
• AGENTS.md streamlined to reference skills