Refs #8933: validate third-domain onboarding dashboard flow

[T-rav]

Summary

• persist the onboarding draft current plan when Continue to next plan files Plan N+1 issues
• add a third-domain validation regression for an Acme/claims-ledger onboarded repo after push and Plan 02 continuation
• verify the dashboard-facing repo list, repo-scoped event stream, repo metrics, and generated hydraflow-find issue bodies from one route surface

Verification

• uv run pytest tests/test_onboarding_api.py tests/test_dashboard_routes_metrics.py::TestMetricsEndpoint::test_metrics_includes_repo_scoped_throughput_and_friction tests/test_dashboard_routes_state.py::TestGetEventsEndpoint::test_repo_query_uses_repo_runtime_event_bus -q
• npm test -- --run src/components/tests/ProjectView.test.jsx src/context/tests/HydraFlowContext.test.jsx src/components/tests/MetricsPanel.test.jsx src/components/tests/RepoSelector.test.jsx
• uv run pyright src/onboarding/models.py src/dashboard_routes/_onboarding_routes.py tests/test_onboarding_api.py
• make quality