docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+## Maintainer Expectations
+
+Keep changes small, reviewable, and tied to a clear project purpose. Do not mix source changes with generated build output or dependency caches.
+
+## Before Committing
+
+- Run the relevant build, lint, or smoke test when the project provides one.
+- Check that no credentials, `.env` files, tokens, private keys, captures, dumps, or personal data are staged.
+- Keep firmware binaries, large archives, and generated artifacts out of Git unless the repo explicitly documents otherwise.
+- Preserve upstream licenses and attribution for third-party code.
+
+## Documentation
+
+Update README and wiki pages when setup, hardware, architecture, environment variables, or release behavior changes.
+
+## Safety
+
+Only submit work intended for authorized environments. Project documentation should make scope and safe operation clearer, never weaker.