Replace D2 rendering with DiagramAgent (Mermaid + Kroki)

- Delete d2-tool.ts and D2_RENDER_URL references
- Add diagram-agent.ts: OpenAI generates Mermaid → Kroki renders PNG → Telegram sendPhoto + R2 cache
- Update onboarding-agent to use generate_diagram tool with DiagramAgent
- Switch wrangler.toml from D2_RENDER_URL to KROKI_URL
- Remove D2 from env-validation, api-urls, tools/index

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

src/utils/api-urls.ts

@@ -1,8 +1,6 @@
 import type { Env } from '../types';
 
 const DEFAULT_OPENAI_GATEWAY = 'https://gateway.ai.cloudflare.com/v1/d8e5997eb4040f8b489f09095c0f623c/telegram-ai-support/openai';
-const DEFAULT_D2_RENDER_URL = 'http://10.253.100.107:8080';
-
 /**
  * OpenAI Chat Completions API URL (AI Gateway 경유)
  */
@@ -18,9 +16,3 @@ export function getOpenAIBaseUrl(env: Env): string {
   return env.OPENAI_API_BASE || DEFAULT_OPENAI_GATEWAY;
 }
 
-/**
- * D2 diagram rendering service URL
- */
-export function getD2RenderUrl(env: Env): string {
-  return env.D2_RENDER_URL || DEFAULT_D2_RENDER_URL;
-}