docs(testing): add comprehensive guide_testing.md

Comprehensive guide covering the 251-test-file suite:
- Test file layout and naming conventions
- 7 conftest.py fixtures (isolate_workspace, reset_paths, reset_ai_client, vlogger, kill_process_tree, mock_app, app_instance, live_gui) with their mechanisms
- 5 test categories (unit, integration, mock app, headless, opt-in)
- Markers (integration, clean_install, docker) and how to filter by them
- Hook API for integration tests (ApiHookClient methods, predefined_callbacks pattern)
- Common test patterns (pure function, mock, live_gui, exception, parametrized)
- Test configuration in pyproject.toml
- Running tests (all, by file, by marker, with timeout, etc.)
- Adding new tests (pure, integration, opt-in)
- Debugging failed tests (common failure modes and fixes)
- The check_test_toml_paths.py audit script
- Test data flow diagram

config.toml

@@ -38,7 +38,7 @@ separate_external_tools = false
 "AI Settings" = true
 "MMA Dashboard" = false
 "Task DAG" = false
-"Usage Analytics" = true
+"Usage Analytics" = false
 "Tier 1" = false
 "Tier 2" = false
 "Tier 3" = false