Add AGENTS.md

[BolajiOlajide]

Adds an AGENTS.md at the repository root so coding agents have accurate context when working here.

It documents the layout, make build / make test, the ./test script's PASSES system and its available passes, and the PASSES='fmt' ./test lint entry point.

It also calls out two traps: gofmt is only checked rather than applied, and several linters are silently skipped when their binaries are absent, so a clean local run does not imply a clean CI run. It notes that this is a mirror of etcd-io/etcd and that the deliberate replace directive pinning zap should be left alone.

The commit title uses the *: prefix required by this repository's commit_title_pass check.

Created by a Sourcegraph agentic batch change.