docs: add AGENTS.md with agent instructions

[eqdmc-admin]

What

Add an AGENTS.md file at the repo root per the LF agents-md v1 spec. This tells agent tools (Claude Code, OpenCode, Codex, Cursor) how to work in this repository.

Acceptance checklist

• AGENTS.md exists at repo root
• Contains: repo purpose, tech stack, how to run tests, how to build
• References CLAUDE.md if it exists
• File is under 50 lines

Verification

test -f AGENTS.md && wc -l AGENTS.md