feat: add OpenAPI 3.1 spec and MCP server binary

[ZPascal]

Summary

• Static OpenAPI 3.1 spec embedded in binary, served at GET /openapi.json (no auth required)
• Documents all endpoints: POST /register, POST /update, GET /health, and the admin record CRUD endpoints
• New cmd/acme-dns-mcp/ binary: stdio-based MCP server wrapping the acme-dns HTTP API as structured tools
• MCP tools: register_subdomain, update_txt_record, list_dns_records, create_dns_record, update_dns_record, delete_dns_record, health_check
• Config loaded from ~/.acme-dns-mcp/config.toml or env vars (ACMEDNS_BASE_URL, ACMEDNS_ADMIN_TOKEN, etc.)
• Credentials never passed as tool arguments — config only
• .goreleaser.yml updated to build and publish acme-dns-mcp alongside acme-dns

Test Plan

• go test -race ./... passes
• go build ./... builds cleanly
• GET /openapi.json returns valid OpenAPI 3.1 JSON
• MCP health_check tool returns {"status":"ok"} against a running instance
• MCP list_dns_records returns {"records":[...]} with admin token configured
• Missing credentials return {"error":"admin_token not configured"}