fix: Add library export fields to npm package (main, types, exports)

## Problem
PR #309 added `main`, `types`, and `exports` fields to package.json for library usage,
but v2.18.9 was published without these fields. The publish scripts (both local and CI/CD)
use package.runtime.json as the base and didn't copy these critical fields.

Result: npm package broke library usage for multi-tenant backends.

## Root Cause
Both scripts/publish-npm.sh and .github/workflows/release.yml:
- Copy package.runtime.json as base package.json
- Add metadata fields (name, bin, repository, etc.)
- Missing: main, types, exports fields

## Changes

### 1. scripts/publish-npm.sh
- Added main, types, exports fields to package.json generation
- Removed test suite execution (already runs in CI)

### 2. .github/workflows/release.yml
- Added main, types, exports fields to CI publish step

### 3. Version bump
- Bumped to v2.18.10 to republish with correct fields

## Verification
✅ Local publish preparation tested
✅ Generated package.json has all required fields:
   - main: "dist/index.js"
   - types: "dist/index.d.ts"
   - exports: { "." : { types, require, import } }
✅ TypeScript compilation passes
✅ All library export paths validated

## Impact
- Fixes library usage for multi-tenant deployments
- Enables downstream n8n-mcp-backend project
- Maintains backward compatibility (CLI/Docker unchanged)

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

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

package.runtime.json

@@ -1,6 +1,6 @@
 {
   "name": "n8n-mcp-runtime",
-  "version": "2.18.7",
+  "version": "2.18.10",
   "description": "n8n MCP Server Runtime Dependencies Only",
   "private": true,
   "dependencies": {