- Add backend onboarding session service and legacy API routes for start, state, and import/export generation
- Introduce experimental AgentOnboardingModal UI with stream-state handling, model selector wiring, and onboarding launch flow in AgentsView
- Add onboarding gating via prompt overrides/store updates and include search behavior fix for partial-match filtering
- Expand dashboard/core tests for onboarding API, modal behavior, agents/settings integration, and prompt override handling
Fusion-Task-Id: FN-3025
