feat: add OpenAPI 3.1 spec and MCP server binary

[ZPascal]

Summary

• Static openapi.json (OpenAPI 3.1) documenting all endpoints served at GET /openapi.json via go:embed
• New acme-dns-mcp binary in cmd/acme-dns-mcp/ — stdio MCP server compatible with Claude Code (/mcp add) and any MCP-capable agent
• 7 MCP tools: health_check, register_subdomain, update_txt_record, list_dns_records, create_dns_record, update_dns_record, delete_dns_record
• Config via ~/.acme-dns-mcp/config.toml or env vars (ACMEDNS_BASE_URL, ACMEDNS_ADMIN_TOKEN, ACMEDNS_USERNAME, ACMEDNS_PASSWORD)
• Credentials never passed as tool arguments — config only
• goreleaser updated to publish both binaries in releases

Depends on: #222

Test Plan

• go test ./... passes (including cmd/acme-dns-mcp)
• go build ./... builds both binaries
• GET /openapi.json returns 200 with valid OpenAPI 3.1 JSON containing "openapi" key
• echo '{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}' | acme-dns-mcp returns 7 tools
• echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{}}' | acme-dns-mcp returns protocolVersion: "2024-11-05"
• Admin tools return {"error":"admin_token not configured"} when token not set