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>

.gitignore

@@ -90,3 +90,19 @@ rebuild-v20.sh
 # n8n-docs repo (cloned locally)
 ../n8n-docs/
 n8n-docs/
+
+# npm publish temporary directory
+npm-publish-temp/
+
+# Test files and logs
+test-npx/
+mcp-server-*.log
+server.log
+server-fixed.log
+mcp-debug.log
+
+# Temporary wrapper scripts
+n8n-mcp-wrapper.sh
+
+# Package tarballs
+*.tgz