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>

.github/workflows/release.yml

@@ -334,6 +334,15 @@ jobs:
           const pkg = require('./package.json');
           pkg.name = 'n8n-mcp';
           pkg.description = 'Integration between n8n workflow automation and Model Context Protocol (MCP)';
+          pkg.main = 'dist/index.js';
+          pkg.types = 'dist/index.d.ts';
+          pkg.exports = {
+            '.': {
+              types: './dist/index.d.ts',
+              require: './dist/index.js',
+              import: './dist/index.js'
+            }
+          };
           pkg.bin = { 'n8n-mcp': './dist/mcp/index.js' };
           pkg.repository = { type: 'git', url: 'git+https://github.com/czlonkowski/n8n-mcp.git' };
           pkg.keywords = ['n8n', 'mcp', 'model-context-protocol', 'ai', 'workflow', 'automation'];