fix: revert to Node 20 and use granular NPM token

NPM classic tokens were revoked on Dec 9, 2025. OIDC trusted publishing
requires npm >= 11.5.1 which caused lockfile sync issues with npm ci.

Reverted to Node 20 with granular access token approach:
- Removed OIDC permissions block
- Removed npm upgrade step
- Restored NODE_AUTH_TOKEN usage
- Removed --provenance flag

User created new granular token with "Bypass 2FA" enabled.

Conceived by Romuald Członkowski - www.aiadvisors.pl/en

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.runtime.json

@@ -1,6 +1,6 @@
 {
   "name": "n8n-mcp-runtime",
-  "version": "2.29.4",
+  "version": "2.29.5",
   "description": "n8n MCP Server Runtime Dependencies Only",
   "private": true,
   "dependencies": {