Add CONTRIBUTING.md with branch protection and development workflow

CONTRIBUTING.md

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+# Contributing to ArtifactLens
+
+## Branch Protection
+
+The `main` branch is protected. Direct pushes are not allowed.
+
+All changes must go through a pull request:
+
+1. **Create a feature branch** from `main`
+   ```bash
+   git checkout main
+   git pull
+   git checkout -b your-branch-name
+   ```
+
+2. **Make your changes**, commit, and push
+   ```bash
+   git add .
+   git commit -m "Describe your change"
+   git push origin your-branch-name
+   ```
+
+3. **Open a Pull Request** on GitHub against `main`
+
+4. **Wait for CI** — the pipeline must pass (syntax check + build)
+
+5. **Request review** — a project maintainer must approve before merging
+
+## CI Pipeline
+
+Every push and PR triggers the CI workflow which:
+- Runs syntax checks on all Python source files
+- Validates that all modules import correctly
+- Builds the executable via PyInstaller
+- Uploads the build artifact
+
+PRs cannot be merged until CI passes.
+
+## Development Setup
+
+```bash
+# Clone the repo
+git clone https://github.com/william051200/azure-artifacts-search.git
+cd azure-artifacts-search
+
+# Create a virtual environment
+python -m venv venv
+venv\Scripts\activate
+
+# Install dependencies
+pip install -r requirements.txt
+```
+
+## Running the App
+
+```bash
+python -m search_artifact_app
+```
+
+## Building the Executable
+
+```bash
+build.bat
+```
+
+The output will be at `dist\ArtifactLens\ArtifactLens.exe`.