docs(integrations): add Zapier and n8n docs; align n8n toolchain and codex

- Add docs/integrations/zapier.mdx and n8n.mdx, register both in docs.json
  (Developer Tools) and docs/llms.txt
- Align n8n CD toolchain (pnpm 9 / Node 20) with the checks workflow so the
  published artifact is built on the toolchain CI validated
- Add Mem0.node.json codex and copy it into dist via gulp for nodes-panel
  discoverability
- Align search default limit to 50 across Zapier and n8n

.github/workflows/n8n-nodes-mem0-cd.yml

@@ -34,12 +34,12 @@ jobs:
       - name: Install pnpm
         uses: pnpm/action-setup@v4
         with:
-          version: 10
+          version: 9
 
       - name: Set up Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: '22'
+          node-version: '20'
           registry-url: 'https://registry.npmjs.org'
           cache: 'pnpm'
           cache-dependency-path: integrations/n8n-nodes-mem0/pnpm-lock.yaml