docs: add AGENTS.md contributor-safe guidance

Author: timothytlewis

AGENTS.md

@@ -0,0 +1,25 @@
+# AGENTS: pkgx
+
+Public core CLI repository for running packages.
+
+## Core Commands
+
+- `cargo fmt --all -- --check`
+- `cargo clippy --all-features`
+- `cargo test --all-features`
+
+## Always Do
+
+- Preserve CLI semantics and user-facing flags unless intentionally versioned.
+- Keep changes in `crates/lib` and `crates/cli` compatibility-aware.
+- Add tests for changed runtime behavior.
+
+## Ask First
+
+- Breaking CLI behavior changes.
+- Distribution/release workflow changes.
+
+## Never Do
+
+- Never skip tests on core execution path changes.
+- Never merge compatibility-sensitive changes without impact notes.