Pin pnpm via packageManager

pnpm/action-setup@v4 now hard-requires an explicit version — none of
ci.yml, integration.yml, or release.yml pinned one, so every workflow run
here has been failing at the setup step (confirmed via gh run list, not
just release.yml's new dry run). packageManager is the field
pnpm/action-setup reads automatically, fixing all three workflows from
one place instead of patching each one's action-setup step.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: shixish

package.json

@@ -3,6 +3,7 @@
   "version": "0.2.0",
   "description": "LangChain integration for the Diffbot APIs — Knowledge Graph, Extract, Web Search, NLP entities, Crawl, and the Diffbot LLM RAG endpoint",
   "type": "module",
+  "packageManager": "pnpm@10.33.0",
   "main": "./dist/index.cjs",
   "module": "./dist/index.js",
   "types": "./dist/index.d.ts",