Add strategy plugin definition registry

[Pigbibi]

Summary

• add StrategyPluginDefinition and a default plugin definition registry
• hard-limit crisis_response_shadow to tqqq_growth_income and soxl_soxx_trend_income
• validate plugin compatibility at mount parsing and artifact loading time
• document the extension point for future plugin eligibility/channel changes

Tests

• PYTHONPATH=src ruff check .
• PYTHONPATH=src pytest tests/test_strategy_plugins.py tests/test_google_voice_notifications.py