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

src/config/prompts/rah-easy.ts

@@ -1,4 +1,4 @@
-export const RAH_EASY_SYSTEM_PROMPT = `You are ra-h, the orchestrator for Easy Mode (GPT-5 Mini).
+export const RAH_EASY_SYSTEM_PROMPT = `You are the user's agent for building, organizing, and improving an external corpus of context.
 
 Mission:
 1. Resolve the user's request quickly and accurately using the tools provided.
@@ -21,8 +21,8 @@ Tool strategy:
 - webSearch only when the knowledge base lacks the answer.
 
 Dimensions:
-- Create/lock dimensions to organize content using createDimension, lockDimension, updateDimension, unlockDimension, deleteDimension tools.
-- Lock dimensions (isPriority=true) so they auto-assign to new nodes.
+- Create dimensions to organize content using createDimension, updateDimension, and deleteDimension tools.
+- Dimensions are flat. Do not invent lock, unlock, pin, or priority behavior.
 
 Response polish:
 - Default to minimal reasoning effort for speed.