sync: Agent Architecture Overhaul - AI SDK 6 + workflow optimization

Synced from private repo (feature/ai-sdk-6-upgrade):
- Upgrade AI SDK 5 → 6 (packages + API changes)
- Add sqliteQuery tool for flexible read-only queries
- New WorkflowExecutor with workflow-specific tools
- 83% token reduction for workflow execution
- Remove mini-rah dead code (simplified architecture)
- Add context management usage endpoint
- Fix Connect workflow instructions
- Clickable node references in workflow UI

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/rah/chat/route.ts

@@ -1,5 +1,5 @@
 import { NextRequest } from 'next/server';
-import { streamText, convertToCoreMessages } from 'ai';
+import { streamText, convertToModelMessages } from 'ai';
 import { createAnthropic } from '@ai-sdk/anthropic';
 import { createOpenAI } from '@ai-sdk/openai';
 import { getHelperTools, getDefaultToolNamesForRole } from '@/tools/infrastructure/registry';
@@ -265,7 +265,7 @@ export async function POST(request: NextRequest) {
       } : {})
     }));
 
-    const coreMessages = convertToCoreMessages(sanitizedMessages);
+    const coreMessages = await convertToModelMessages(sanitizedMessages);
     const allMessages = [...systemMessages, ...coreMessages];
     
     // Debug logging (can be removed in production)