add developer guidelines for coding style and pull requests

CONTRIBUTING.md

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+# Contributing to SecureLens
+
+Welcome to SecureLens! We appreciate your interest in contributing to the project. Follow these guidelines to ensure a smooth collaboration process.
+
+---
+
+## Code of Conduct
+
+By participating in this project, you agree to abide by our Code of Conduct. Please read `CODE_OF_CONDUCT.md` to understand the expectations we have for community members.
+
+---
+
+## How to Contribute
+
+### 1. Report Issues and Propose Features
+
+Before writing code, search our issue tracker to ensure the topic has not been discussed or resolved. If you find a new bug or have a feature request:
+* Create a detailed issue explaining the bug, with steps to reproduce it.
+* Describe the expected behavior and system details (OS, Python version, configuration).
+* If proposing a new feature, explain the use case and how it benefits developers.
+
+### 2. The Pull Request Process
+
+To submit code changes:
+1. Fork the repository on GitHub.
+2. Clone your fork locally.
+3. Create a descriptive feature branch from `main`:
+   ```bash
+   git checkout -b feature/your-feature-name
+   ```
+4. Write your code, update relevant documentation, and write unit tests.
+5. Run the test suite and verify all checks pass.
+6. Commit your changes with clear, structured messages.
+7. Push to your fork and submit a Pull Request (PR) to our `main` branch.
+
+---
+
+## Local Development and Style Standards
+
+### 1. Code Style
+
+We enforce standard Python formatting to keep the codebase maintainable and readable:
+* Use standard line length limits (79 or 88 characters).
+* Use explicit type hinting for function arguments and return values where possible.
+* Document modules, classes, and public functions with clear docstrings.
+
+### 2. Linting and Formatting
+
+Run linting checks before committing. If you use formatters like `black` or `ruff`:
+```bash
+# Format the code base
+black app/ cli/ tests/
+
+# Run static analysis
+ruff check app/ cli/ tests/
+```
+
+We recommend installing the pre-commit hook framework to automate checks (see `.pre-commit-config.yaml`).
+
+### 3. Running the Test Suite
+
+We use `pytest` for unit and integration testing. All tests are located under the `tests/` directory.
+
+Run the entire test suite locally:
+```bash
+pytest tests/ -v
+```
+
+If you are modifying the local CLI scanner, run specific tests:
+```bash
+pytest tests/test_cli_patterns.py -v
+pytest tests/test_cli_sync.py -v
+pytest tests/test_cli_pdf.py -v
+```
+
+Ensure all tests pass and write new tests covering any logic changes or new features you introduce.
+
+---
+
+## Pull Request Guidelines
+
+To help maintainers review your PR efficiently:
+* Keep PRs focused. Do not combine multiple unrelated changes into one pull request.
+* Write a clear title and description explaining what was changed and why.
+* Reference any related issues (e.g., `Closes #123`).
+* Verify that your branch is rebased on top of the latest `main` branch.