- Create tests/plugins/ with auto-discovery fixtures and generic tests
- Add conftest.py with plugin_ids, loaded_plugin, host_context fixtures
- Add test_all_plugins.py with 3 test classes:
- TestAllPluginsManifest: validates manifest structure
- TestAllPluginsEngine: validates engine lifecycle contract
- TestAllPluginsCapabilities: validates capability implementation
- Update docs/testing.md with auto-discovery documentation
- Plugin-specific tests remain in tests/test_*_plugin.py for integration
New plugins in plugins/ are now automatically tested without manual test creation.
