chore(integrations): use pnpm, real Mem0 icon, fix Zapier field defaults

- Switch both packages from npm to pnpm (pnpm-lock.yaml) to match the
  repo convention (openclaw / pi-agent-plugin / vercel-ai-sdk); gitignore
  package-lock.json.
- Replace the placeholder n8n node icon with the real Mem0 mark (same
  asset used by integrations/mem0-plugin).
- Zapier: revert boolean/integer field defaults to strings — Zapier's
  schema requires string defaults (zapier validate now passes with 0
  errors). Runtime coercion (String(x)!=='false', Number()) is unchanged.

integrations/n8n-nodes-mem0/.gitignore

@@ -1,5 +1,6 @@
 node_modules/
 dist/
+package-lock.json
 *.tsbuildinfo
 .env
 coverage/