Clarify supported Python setup - #7

Summary

• make README and Getting Started activation commands work on macOS/Linux and Windows PowerShell
• document the Python 3.11+ requirement and the Python 3.11/3.12 CI matrix
• use platform-appropriate commands for copying the optional .env file

Validation

• git diff --check
• python3 -m ruff check src tests scripts
• python3 -m black --check src tests scripts
• python3 -m mypy src --ignore-missing-imports
• python3 -m mkdocs build --strict
• Full pytest is covered by CI's Python 3.11/3.12 matrix; the local environment only provides Python 3.10, where collection fails on the project's enum.StrEnum requirement.