feat(engine): add plugin integration layer for cortex-plugins hooks

Add PluginIntegration struct that bridges cortex-engine and cortex-plugins
hook systems, enabling:
- Tool execution hooks (before/after)
- Session lifecycle hooks (start/end)
- Permission decision hooks
- Chat message hooks

Includes PluginIntegrationBuilder for flexible construction and
comprehensive test coverage for all hook trigger methods.

Author: echobt