Add CONTRIBUTING.md with fork and branch workflow

CONTRIBUTING.md

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+# Contributing to TemGymCore
+
+Thank you for your interest in contributing! This guide explains how to fork the repository, create a branch, and submit your changes.
+
+## Forking and Branching
+
+### 1. Fork the repository
+
+1. Navigate to [https://github.com/TemGym/TemGymCore](https://github.com/TemGym/TemGymCore) in your browser.
+2. Click the **Fork** button in the top-right corner.
+3. Select your GitHub account as the destination. GitHub will create a copy of the repository at `https://github.com/<your-username>/TemGymCore`.
+
+### 2. Clone your fork locally
+
+```bash
+git clone https://github.com/<your-username>/TemGymCore.git
+cd TemGymCore
+```
+
+### 3. Add the upstream remote
+
+Keep your fork up to date with the original repository by adding it as a remote:
+
+```bash
+git remote add upstream https://github.com/TemGym/TemGymCore.git
+```
+
+Verify the remotes:
+
+```bash
+git remote -v
+# origin    https://github.com/<your-username>/TemGymCore.git (fetch)
+# origin    https://github.com/<your-username>/TemGymCore.git (push)
+# upstream  https://github.com/TemGym/TemGymCore.git (fetch)
+# upstream  https://github.com/TemGym/TemGymCore.git (push)
+```
+
+### 4. Create a new branch
+
+Always create a new branch for your changes rather than working on `main`:
+
+```bash
+git checkout -b my-feature-branch
+```
+
+Use a descriptive branch name, e.g. `fix-lens-propagation` or `add-new-component`.
+
+### 5. Make your changes and commit
+
+```bash
+# ... edit files ...
+git add .
+git commit -m "Short description of your change"
+```
+
+### 6. Push your branch to your fork
+
+```bash
+git push origin my-feature-branch
+```
+
+### 7. Open a Pull Request
+
+1. Go to your fork on GitHub (`https://github.com/<your-username>/TemGymCore`).
+2. Click **Compare & pull request** next to your branch.
+3. Fill in the title and description, then click **Create pull request**.
+
+## Keeping your fork up to date
+
+Before starting new work, sync your fork's `main` branch with upstream:
+
+```bash
+git checkout main
+git fetch upstream
+git merge upstream/main
+git push origin main
+```
+
+## Development Setup
+
+Install the package in editable mode with development dependencies:
+
+```bash
+pip install -e ".[dev]"
+```
+
+Run the test suite:
+
+```bash
+pytest
+```
+
+Run the linter:
+
+```bash
+flake8 src/
+```
+
+## Questions
+
+If you have questions, feel free to open an [issue](https://github.com/TemGym/TemGymCore/issues).