docs(contributing): Add contributing guide

why: Project had no contributing guidelines for new contributors.
what:
- Add .github/contributing.md with ruff, mypy, pytest, uv workflow
- Reference AGENTS.md and developing docs

Author: tony

.github/contributing.md

@@ -0,0 +1,27 @@
+# Contributing
+
+When contributing to this repository, please first discuss the change you wish to make via issue,
+email, or any other method with the maintainers of this repository before making a change.
+
+See [developing](../docs/developing.md) for environment setup and [AGENTS.md](../AGENTS.md) for
+detailed coding standards.
+
+## Pull Request Process
+
+1. **Format and lint**: `uv run ruff format .` then `uv run ruff check . --fix --show-fixes`
+2. **Type check**: `uv run mypy`
+3. **Test**: `uv run pytest` — all tests must pass before submitting
+4. **Document**: Update docs if your change affects the public interface
+5. You may merge the Pull Request once you have the sign-off of one other developer. If you
+   do not have permission to do that, you may request a reviewer to merge it for you.
+
+## Decorum
+
+- Participants will be tolerant of opposing views.
+- Participants must ensure that their language and actions are free of personal
+  attacks and disparaging personal remarks.
+- When interpreting the words and actions of others, participants should always
+  assume good intentions.
+- Behaviour which can be reasonably considered harassment will not be tolerated.
+
+Based on [Ruby's Community Conduct Guideline](https://www.ruby-lang.org/en/conduct/)