Remove .env config support

[pmbstyle]

Summary

• remove automatic .env loading and migration from the runtime config path
• update docker-compose and docs to use config.json as the only supported file-based config
• drop python-dotenv and remove the legacy .env example

Testing

• uv run pytest tests/test_settings_config_sync.py
• uv run python -m compileall src/octopal/infrastructure/config/settings.py src/octopal/cli/configure.py