docs: add config JSON schema

[haosenwang1018]

Summary

• add a JSON Schema for mcp_servers.json
• document how to reference the schema from editor tooling via $schema
• keep the schema aligned with the current stdio/HTTP config options and tool filtering fields

Problem

Issue #32 asks for a schema file that users can point their editors at to validate mcp_servers.json and get autocomplete / formatting guidance.

Changes

• add mcp_servers.schema.json at the repository root
• cover both stdio and HTTP server definitions, including allowedTools / disabledTools
• document the schema in the README with an example $schema entry

Fixes #32

Validation

• node -e "JSON.parse(require('node:fs').readFileSync('mcp_servers.schema.json', 'utf8'))"
• git diff --check