Add AGENTS.md and migrate CLAUDE.md to it #1005

Claude Code now reads AGENTS.md natively, so we can keep one instructions file that works for every AI tool (Claude Code, Codex, Copilot, Cursor, CodeRabbit).

• Move CLAUDE.md content to AGENTS.md
• Replace CLAUDE.md with a one-line @AGENTS.md import (keeps it working on Claude Code < 2.1.277 and when a contributor has a CLAUDE.local.md, which otherwise makes Claude skip AGENTS.md)
• Update references to CLAUDE.md (.coderabbit.yaml, .claude/skills/contributing/SKILL.md)
• Remove tool-specific wording where it no longer applies

Related: TEDI-Design-System/angular#788, #1005, TEDI-Design-System/core#168