Share AI agent contribution rules across agents via AGENTS.md

The contribution and commit rules previously lived only in .claude/CLAUDE.md,
which Claude Code reads but other agents do not. This project is also worked
on with GitHub Copilot and may be used with Codex, Cursor, Gemini CLI and
others, all of which must follow the same rules — above all that every commit
is authored by a human.

Put the canonical rules in AGENTS.md, the cross-tool standard read natively by
Codex, Cursor, the Copilot coding agent, VS Code, Gemini CLI, Zed, Aider,
Windsurf and many more. The few agents that use a dedicated file load the same
single source from there:

- .claude/CLAUDE.md imports it via Claude Code's @ syntax
- GEMINI.md imports it via Gemini CLI's @ syntax
- .github/copilot-instructions.md points to it

so there is no second copy to keep in sync.

Assisted-by: claude-code:claude-opus-4-8