test,docs: fix all stale references to deleted multi-agent subsystem
Test fixes (6 files, -303 lines):
- Delete app_controller_acp_mount_resilience_test.dart (entirely about deleted types)
- Remove multi-agent test cases from gateway_acp_client_auth_test.dart
- Rename _manifestWithDesktopMultiAgentEnabled → _defaultDesktopManifest
in assistant_execution_target_test, assistant_lower_pane_test,
mobile_assistant_page_test
Docs fixes (6 files):
- Regenerate public-symbol-inventory.json/md via make docs-public-api
- Remove multi-agent sections from public-api/models-and-config.md,
app-orchestration.md, runtime-contracts.md
- Fix xworkmate/ → xworkmate-app/ paths in cloud-session doc
- Remove multiAgent references from app-external-service-api-test-matrix.md
fdda0b7d85