Adopt AGENTS.md as single agent-instruction file (drop copilot mirror)

[jml6m]

Adopt AGENTS.md as the single agent-instruction file

This repo currently has only .github/copilot-instructions.md. Standardize on AGENTS.md (read natively by Copilot, and by Claude Code via a CLAUDE.md that @AGENTS.md-imports).

• Move .github/copilot-instructions.md content → AGENTS.md at repo root.
• Add CLAUDE.md whose body is @AGENTS.md.
• Delete .github/copilot-instructions.md.
• Trim to the minimum relevant content (low-activity repo).

Issues-only this round.