Add Stack Exchange ZIMs and wire Kiwix search into MCP server

- kiwix_download.sh: add Ask Ubuntu, Super User, Unix & Linux SE, Server Fault
- mcp_server.py: add kiwix_search tool — models can now query all offline ZIMs
- local-ai-setup.sh: pass KIWIX_URL env to MCP container, add kiwix dependency

https://claude.ai/code/session_012gDnantBmFTWZGCiKyjazx

local-ai-setup.sh

@@ -612,11 +612,12 @@ services:
       - REPOS_DIR=/repos
       - GITEA_URL=http://gitea:3000
       - RAG_URL=http://rag-server:8001
+      - KIWIX_URL=http://kiwix:80
     command: >
       bash -c "apt-get update -qq && apt-get install -y --no-install-recommends git ripgrep &&
                pip install --no-cache-dir -r mcp_requirements.txt &&
                python mcp_server.py"
-    depends_on: [rag-server]
+    depends_on: [rag-server, kiwix]
 
   kiwix:
     image: ghcr.io/kiwix/kiwix-serve:latest