Stage 16: connector test-env via UseTestEnvironment; pin it in the test contour

- bot.New now selects Telegram's test environment with the library's native
  tgbot.UseTestEnvironment() instead of a token += "/test" hack (functionally
  identical URL /bot<token>/test/METHOD, but idiomatic) + a bot test asserting
  the getMe path for both test and prod.
- ci.yaml pins TELEGRAM_TEST_ENV=true for the test contour (it IS the test
  environment) instead of a TEST_TELEGRAM_TEST_ENV variable: removes the
  confusing double-TEST, telegram-specific, prefixed operator knob and the
  secret-vs-variable footgun. Prod (Stage 17) leaves it false.
- deploy/README.md + PLAN.md updated.

platform/telegram/internal/bot/bot_test.go

@@ -75,6 +75,34 @@ func TestSendTextHasNoMarkup(t *testing.T) {
 	}
 }
 
+// getMePathFor captures the path bot.New's getMe call hits for the given TestEnv,
+// so the test environment routing is covered (a misroute is exactly what makes a
+// test-environment token fail with "getMe unauthorized").
+func getMePathFor(t *testing.T, testEnv bool) string {
+	t.Helper()
+	var path string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if strings.HasSuffix(r.URL.Path, "/getMe") {
+			path = r.URL.Path
+		}
+		io.WriteString(w, `{"ok":true,"result":{"id":1,"is_bot":true,"first_name":"t","username":"tb"}}`)
+	}))
+	t.Cleanup(srv.Close)
+	if _, err := New(Config{Token: "123:ABC", APIBaseURL: srv.URL, TestEnv: testEnv, MiniAppURL: "https://example.com/"}, zap.NewNop()); err != nil {
+		t.Fatalf("new bot (testEnv=%v): %v", testEnv, err)
+	}
+	return path
+}
+
+func TestTestEnvironmentRoutesGetMe(t *testing.T) {
+	if got, want := getMePathFor(t, true), "/bot123:ABC/test/getMe"; got != want {
+		t.Errorf("TestEnv getMe path = %q, want %q", got, want)
+	}
+	if got, want := getMePathFor(t, false), "/bot123:ABC/getMe"; got != want {
+		t.Errorf("prod getMe path = %q, want %q", got, want)
+	}
+}
+
 func TestStartPayload(t *testing.T) {
 	cases := map[string]string{
 		"/start g123":  "g123",