general: T8595: add AGENTS.md

[andamasov]

Adds AGENTS.md (tool-neutral primary instructions) at repo root and a symlink .github/copilot-instructions.md → ../AGENTS.md for Copilot code review compatibility.

Schema rationale and full spec: T8595.

Why this shape: Cursor 0.50+, Claude Code, OpenAI Codex, and Copilot Coding Agent / Chat / CLI read AGENTS.md natively. Copilot code review is the single tool that doesn't (per GitHub's docs) — the symlink is the workaround until community/discussions/174058 lands. Replaces the previous CLAUDE.md-only schema PR for this repo.

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