feat(FN-3025): add agent onboarding flow with launch gating

- 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

packages/core/src/__tests__/prompt-overrides.test.ts

@@ -82,13 +82,14 @@ describe("prompt-overrides", () => {
   describe("getPromptKeysForRole", () => {
     it("should return all keys for executor role", () => {
       const keys = getPromptKeysForRole("executor");
-      expect(keys).toHaveLength(8);
+      expect(keys).toHaveLength(9);
       expect(keys.map((k) => k.key)).toContain("executor-welcome");
       expect(keys.map((k) => k.key)).toContain("executor-guardrails");
       expect(keys.map((k) => k.key)).toContain("executor-spawning");
       expect(keys.map((k) => k.key)).toContain("executor-completion");
       expect(keys.map((k) => k.key)).toContain("agent-generation-system");
       expect(keys.map((k) => k.key)).toContain("workflow-step-refine");
+      expect(keys.map((k) => k.key)).toContain("agent-onboarding-system");
       expect(keys.map((k) => k.key)).toContain("subtask-breakdown-system");
       expect(keys.map((k) => k.key)).toContain("ai-refine-system");
     });