chore: add AGENTS.md for AI coding assistants (#664)

Author: bnusunny

AGENTS.md

@@ -0,0 +1,22 @@
+# AGENTS.md
+
+## Build & Test
+
+- Build: `cargo build`
+- Run all tests: `cargo nextest run`
+- Run a single test: `cargo nextest run <test_name>`
+- Lint: `cargo clippy --all-targets`
+- Docs: `cargo doc --no-deps`
+
+## Project Structure
+
+- `src/lib.rs` — Core adapter logic, public API (`Adapter`, `AdapterOptions`, `Protocol`, `LambdaInvokeMode`)
+- `src/main.rs` — Binary entrypoint
+- `src/readiness.rs` — Readiness check checkpoint timing
+- `tests/integ_tests/` — Integration tests
+- `tests/e2e_tests/` — End-to-end tests (marked `#[ignore]`, require external setup)
+
+## Key Conventions
+
+- Environment variables use `AWS_LWA_` prefix
+- Tests that mutate env vars rely on nextest's per-test process isolation