docs: add AGENTS.md contributor-safe guidance

Author: timothytlewis

AGENTS.md

@@ -0,0 +1,25 @@
+# AGENTS: dev
+
+Public repository for developer environment activation tooling.
+
+## Core Commands
+
+- `deno fmt --check .`
+- `deno lint .`
+- `deno check ./app.ts`
+- `deno test --allow-read --allow-write --allow-env`
+
+## Always Do
+
+- Keep activation behavior deterministic across shells and platforms.
+- Preserve compatibility with `pkgm` and `libpkgx` interactions.
+
+## Ask First
+
+- Changes to GitHub Action interfaces.
+- Changes to environment variable behavior with broad downstream impact.
+
+## Never Do
+
+- Never introduce hidden global side effects during activation.
+- Never weaken environment isolation guarantees.