docs: add AGENTS.md contributor-safe guidance

Author: timothytlewis

AGENTS.md

@@ -0,0 +1,24 @@
+# AGENTS: cargox
+
+Public repository for Cargo ecosystem command execution tooling.
+
+## Core Commands
+
+- `cargo fmt --check`
+- `cargo clippy --all-targets --all-features -- -D warnings`
+- `cargo test`
+
+## Always Do
+
+- Keep install and version resolution behavior stable.
+- Preserve clear error messaging for install fallback paths.
+
+## Ask First
+
+- Changes to release script semantics (`scripts/release.py`).
+- Any default behavior change for install backend selection.
+
+## Never Do
+
+- Never hide install failures.
+- Never remove safety checks around command invocation.