Consolidate dev dependencies in pyproject.toml and eliminate redundancy

[Copilot]

Addresses feedback on PR #53 regarding duplicate dependency declarations between requirements-dev.txt and pyproject.toml[project.optional-dependencies.dev], which caused redundant installations and potential version conflicts.

Changes

• Added docs extra in pyproject.toml: Separated Sphinx documentation dependencies from dev tools
• Refactored requirements-dev.txt: Now references pyproject.toml extras via -e .[dev,docs] instead of duplicating dependency declarations
• Updated setup scripts: Both setup_dev.sh and setup_dev.bat now install .[dev,docs] directly, eliminating the two-step installation process

Before

# setup_dev.sh installed dependencies twice
pip install -e ".[dev]" --quiet
pip install -r requirements-dev.txt --quiet  # Reinstalled pytest, black, mypy, flake8, isort + sphinx

After

# Single installation with clear separation
pip install -e ".[dev,docs]" --quiet

All dependencies now have a single source of truth in pyproject.toml. The requirements-dev.txt file remains for backwards compatibility but references the pyproject.toml extras.

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