Embed Crawl4AI at all tiers, restore qwen3:4b medium, update docs

- Pre-routing URL fetch: any message with URLs gets content fetched
  async (httpx.AsyncClient) before routing via _fetch_urls_from_message()
- URL context and memories gathered concurrently with asyncio.gather
- Light tier upgraded to medium when URL content is present
- url_context injected into system prompt for medium and complex agents
- Complex agent retains web_search/fetch_url tools + receives pre-fetched content
- Medium model restored to qwen3:4b (was temporarily qwen2.5:1.5b)
- Unit tests added for _extract_urls
- ARCHITECTURE.md: added Tool Handling, Crawl4AI Integration, Memory Pipeline sections
- CLAUDE.md: updated request flow and Crawl4AI integration docs

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

docker-compose.yml

@@ -25,7 +25,7 @@ services:
       - BIFROST_URL=http://bifrost:8080/v1
       # Direct Ollama GPU URL — used only by VRAMManager for flush/prewarm
       - OLLAMA_BASE_URL=http://host.docker.internal:11436
-      - DEEPAGENTS_MODEL=qwen2.5:1.5b
+      - DEEPAGENTS_MODEL=qwen3:4b
       - DEEPAGENTS_COMPLEX_MODEL=qwen3:8b
       - DEEPAGENTS_ROUTER_MODEL=qwen2.5:1.5b
       - SEARXNG_URL=http://host.docker.internal:11437