feat: add Prompt template library, agent_call inter-agent tool, and RAG memory

- New PromptTemplatePicker component for browsing 13 preset prompt templates
- AgentConfig.vue: "Load from library" button for system prompt
- Agents.vue: "Create from Prompt template" entry with agent node + RAG memory
- seed_prompt_templates.py: 13 preset templates (客服/研发/教育/内容/分析/创意/健康医疗)
- agent_call tool: agents can delegate tasks to other agents (19th builtin tool)
- Created 全能助手 (general orchestrator) and 家庭医生助手 agents
- Switch template-created agents from type:llm to type:agent for full ReAct + RAG

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/utils/agentSkills.ts

@@ -14,7 +14,8 @@ export const BUILTIN_SKILL_OPTIONS: { name: string; label: string }[] = [
   { name: 'system_info', label: '系统信息' },
   { name: 'json_process', label: 'JSON 处理' },
   { name: 'database_query', label: '数据库查询' },
-  { name: 'adb_log', label: 'ADB 日志' }
+  { name: 'adb_log', label: 'ADB 日志' },
+  { name: 'agent_call', label: '调用 Agent' },
 ]
 
 export const BUILTIN_SKILL_LABELS: Record<string, string> = Object.fromEntries(