Send SMS with Go

Add SMS sending to this Go project with TextKit.

- Docs for this stack: https://jointextkit.com/docs/send-with-go.md
- API: POST https://api.jointextkit.com/v1/messages with `Authorization: Bearer
  <key>` and a JSON body `{"to": "+1...", "body": "..."}`.
- There is no Go module: POST JSON to https://api.jointextkit.com/v1/messages
  with net/http.
- Read the key from the `TEXTKIT_API_KEY` environment variable. Never hardcode
  it or send it to the browser.
- Phone numbers are E.164 and US or Canada only (`+1` followed by 10 digits).
- A test key (`ssms_sk_test_`) simulates: `+15005550006` always ends `delivered`
  and no phone gets a text.
- Errors are JSON: `{"error": {"code", "message", "param"}}` with a 4xx status.
  On `429`, wait `Retry-After` seconds and retry.
- Send only from server code that runs after a real event (a signup, an order).
  Never expose an endpoint that texts any number a visitor types.

export TEXTKIT_API_KEY=ssms_sk_test_...

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"log"
	"net/http"
	"os"
)

func main() {
	payload, _ := json.Marshal(map[string]string{
		"to":   "+15005550006",
		"body": "Hello from TextKit",
	})

	req, err := http.NewRequest("POST", "https://api.jointextkit.com/v1/messages", bytes.NewReader(payload))
	if err != nil {
		log.Fatal(err)
	}
	req.Header.Set("Authorization", "Bearer "+os.Getenv("TEXTKIT_API_KEY"))
	req.Header.Set("Content-Type", "application/json")

	res, err := http.DefaultClient.Do(req)
	if err != nil {
		log.Fatal(err)
	}
	defer res.Body.Close()

	var out struct {
		ID     string `json:"id"`
		Status string `json:"status"`
		Error  *struct {
			Code    string `json:"code"`
			Message string `json:"message"`
		} `json:"error"`
	}
	if err := json.NewDecoder(res.Body).Decode(&out); err != nil {
		log.Fatal(err)
	}
	if out.Error != nil {
		log.Fatalf("%s: %s", out.Error.Code, out.Error.Message)
	}
	fmt.Println(out.ID, out.Status)
}

go run main.go