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/usage/route.ts

@@ -0,0 +1,28 @@
+import { NextRequest } from 'next/server';
+import { getSQLiteClient } from '@/services/database/sqlite-client';
+
+export async function GET(request: NextRequest) {
+  const sessionId = request.nextUrl.searchParams.get('sessionId');
+
+  if (!sessionId) {
+    return Response.json({ inputTokens: 0 });
+  }
+
+  try {
+    const sqlite = getSQLiteClient();
+    const row = sqlite.prepare(`
+      SELECT json_extract(metadata, '$.input_tokens') as input_tokens
+      FROM chats
+      WHERE thread_id LIKE ?
+      ORDER BY created_at DESC
+      LIMIT 1
+    `).get(`%${sessionId}%`) as { input_tokens: number | null } | undefined;
+
+    return Response.json({
+      inputTokens: row?.input_tokens ?? 0
+    });
+  } catch (error) {
+    console.error('Usage fetch error:', error);
+    return Response.json({ inputTokens: 0 });
+  }
+}