Add AGENTS.md

[BentsiLeviav]

Description

Goal: ensure this repo has a clear, up-to-date AGENTS.md so AI-assisted work is safe and effective.

Acceptance criteria

• AGENTS.md exists at repo root
• Includes exact build/test commands (unit + integration, incl. required env/Docker)
• Documents supported ClickHouse versions and any version-specific behavior (link to a GitHub workflow if needed).
• Describes project structure (key modules + core read/write path)
• Lists hard boundaries (for instance: generated files, protocol/constants, packaging/shading rules, etc.)
• States PR expectations (changelog/versioning/tests required, Lint/Spotless)

Notes

• Keep it concise and executable; prefer commands/examples over prose.
• Link to any existing docs/runbooks instead of duplicating.