Migrate GitHub Copilot instructions to AGENTS.md and CLAUDE.md

Move .github/copilot-instructions.md to AGENTS.md, and generalize the
wording, so it will apply to Codex CLI, Cursor, OpenCode, RooCode, and
many other AI coding agents.

Also add CLAUDE.md as a symlink to AGENTS.md so Claude Code reads the
same guidance.

Copilot coding agent now supports AGENTS.md custom instructions, as
more coding agents support it. Migrating from GitHub Copilot's custom
instructions file to AGENTS.md will help developers leverage more and
different coding agents than just GitHub Copilot.

Reference:
- https://agents.md/
- https://github.blog/changelog/2025-08-28-copilot-coding-agent-now-supports-agents-md-custom-instructions/