Fix: remove references to nonexistent requirements-dev.txt (#123)

GETTING_STARTED.md, CONTRIBUTING.md and PROJECT_STRUCTURE.md instruct
contributors to run 'pip install -r requirements-dev.txt', but that file
does not exist, so setup fails at that step. All development tools
(pytest, pytest-cov, black, flake8) are already provided by
requirements.txt, so the extra install step is removed.
PROJECT_STRUCTURE.md's dev-setup snippet now installs requirements.txt
before the editable install so the documented flow works end to end.

Author: Ayomipo18

CONTRIBUTING.md

@@ -79,7 +79,6 @@ Ready for something meatier? These issues close critical gaps in our production
    source venv/bin/activate
    pip install -r requirements.txt
    pip install -e .
-   pip install -r requirements-dev.txt
    ```
 
 4. **Make your changes**

GETTING_STARTED.md

@@ -18,7 +18,6 @@ source venv/bin/activate  # On Windows: venv\Scripts\activate
 # Install dependencies
 pip install --upgrade pip
 pip install -r requirements.txt
-pip install -r requirements-dev.txt
 
 # Install in editable mode
 pip install -e .

PROJECT_STRUCTURE.md

@@ -280,8 +280,8 @@ Post-processing (inference/)
 
 1. **Set up development environment**
    ```bash
+   pip install -r requirements.txt
    pip install -e .
-   pip install -r requirements-dev.txt
    ```
 
 2. **Run existing code**