chore: Add AGENTS.md and CLAUDE.md

[rekmarks]

Renames .cursorrules to AGENTS.md per the emerging specification, which is supported by Cursor. Also adds a CLAUDE.md that symlinks to AGENTS.md, and git-ignores .claude/settings.local.json.

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Note

Adds contributor guidelines and aligns tooling around Claude-related files.

• New AGENTS.md with coding, testing, architecture, and changelog guidelines
• Adds CLAUDE.md (points to AGENTS.md)
• Updates .gitignore to ignore /.claude/settings.local.json
• Updates package.json:
  • Excludes CLAUDE.md from lint:misc Prettier checks
  • Extends lint-staged pattern to skip formatting for CLAUDE.* files

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