Add CONTRIBUTING.md and instructions for agents

AGENTS.md

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+# AGENTS.md
+
+This file provides guidance to agentic coding assistants like Claude Code (claude.ai/code) when
+working with code in this repository.
+
+## Important: Propose API Changes Before Implementing
+
+Before making or proposing changes to any public API, open a GitHub issue to describe the
+proposal and gather feedback.
+See [CONTRIBUTING.md](./CONTRIBUTING.md) for details.
+
+Do not create pull requests for public API changes without a corresponding issue.

CLAUDE.md

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+@AGENTS.md

CONTRIBUTING.md

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+# Contributing to connect-python
+
+## Before You Contribute
+
+If you're planning to add or change a public API, please open an issue describing your
+proposal before starting work.
+This helps ensure alignment with the project's direction and makes the review process
+smoother for everyone.
+
+## Developer Certificate of Origin
+
+All commits must be signed off to affirm compliance with the
+[Developer Certificate of Origin](https://developercertificate.org/).
+Configure your git identity to match your GitHub account, then use the `-s` flag when
+committing:
+
+```console
+$ git commit -s -m "your commit message"
+```
+
+## Setting Up Your Development Environment
+
+### Prerequisites
+
+- Python 3.10 or later
+- [uv](https://docs.astral.sh/uv/) for dependency management
+- [just](https://just.systems/) for running tasks
+
+### Installation
+
+1. Fork and clone the repository:
+
+   ```console
+   $ gh repo fork connectrpc/connect-python --clone
+   $ cd connect-python
+   ```
+
+2. Verify everything is working:
+
+   ```console
+   $ uv run just check
+   ```
+
+## Development Workflow
+
+We use `just` as our task runner.
+Run `uv run just --list` to see all available commands.
+
+## Submitting a Pull Request
+
+1. Create a feature branch from an up-to-date `main`:
+
+   ```console
+   $ git checkout -b your-feature-branch
+   ```
+
+2. Make your changes and ensure all checks pass:
+
+   ```console
+   $ uv run just check
+   ```
+
+3. Commit with a sign-off and a clear message, then push to your fork and open a pull
+   request.
+
+Pull requests are more likely to be accepted when they:
+
+- Include tests for new functionality
+- Maintain backward compatibility
+- Have clear commit messages
+
+We aim to respond to issues and pull requests within a few business days.