Refs #8933: add onboarding dashboard repo slice

[T-rav]

Summary

• Scope /api/events?repo= to the selected repo runtime event bus so event backfill matches the WebSocket scope.
• Clear stale dashboard metrics/queue/pipeline state when switching repos and route reconnect backfill through the repo-aware fetch helper.
• Show selected-repo plan progress, fleet summary metadata, and next-plan/format-upgrade affordances when project metadata provides them.

Verification

• uv run pytest tests/test_dashboard_routes_state.py::TestGetEventsEndpoint -q
• uv run ruff check src/dashboard_routes/_routes.py tests/test_dashboard_routes_state.py
• uv run pyright src/dashboard_routes/_routes.py tests/test_dashboard_routes_state.py
• cd src/ui && npm test -- src/components/__tests__/ProjectView.test.jsx src/components/__tests__/RepoSelector.test.jsx src/context/__tests__/HydraFlowContext.test.jsx
• cd src/ui && npm test
• cd src/ui && npm run build
• make quality

Notes

This is a dashboard observability slice toward #8933. It does not close the full issue because real Plan 02 execution/issue filing and one-click format-upgrade PR automation still need backend wiring.