feat: sync runtime search and schema quality updates from app repo

- port retrieval, validation, and eval improvements relevant to os
- align prompts and dimensions with the flat single-agent model
- replace the old eval suite with the focused core scenarios

Generated with Codex

tests/evals/scenarios/node-index-search.ts

@@ -0,0 +1,19 @@
+import { Scenario } from '../types';
+
+export const scenario: Scenario = {
+  id: 'node-index-search',
+  name: 'Node index search',
+  description: 'Simple lookup should stay in node search and avoid chunk retrieval.',
+  tools: ['queryNodes'],
+  input: {
+    message: 'Find me the node about Plaintext Productivity. Just return the matching node.',
+  },
+  expect: {
+    toolsCalledSoft: ['queryNodes'],
+    toolsNotCalledSoft: ['searchContentEmbeddings', 'readSkill'],
+    responseContainsSoft: ['Plaintext Productivity'],
+    maxLatencyMs: 15000,
+    maxTotalTokens: 7000,
+    maxEstimatedCostUsd: 0.06,
+  },
+};