feat: add npx support for zero-installation usage (closes #15)

- Add bin configuration to package.json for npx execution
- Implement smart database path resolution for npx/global/local installs
- Create dedicated npm publish script using runtime-only dependencies
- Add .npmignore to control published package contents
- Update README with npx as primary installation method
- Add n8n version badge to README
- Sync version between package.json and package.runtime.json
- Update CHANGELOG for v2.7.8 release

This allows users to run 'npx n8n-mcp' without installing the package,
reducing friction and making it easier to get started with n8n-MCP.

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

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

package.runtime.json

@@ -1,6 +1,6 @@
 {
   "name": "n8n-mcp-runtime",
-  "version": "2.7.0",
+  "version": "2.7.8",
   "description": "n8n MCP Server Runtime Dependencies Only",
   "private": true,
   "dependencies": {