Add terminal package with secure ReadPassword helper

Provides echo-suppressed password prompting via golang.org/x/term for
CLI login commands. Added as a platform standard in engineering-standards.md.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

terminal/terminal_test.go

@@ -0,0 +1,14 @@
+package terminal
+
+import (
+	"testing"
+)
+
+func TestReadPasswordNotATTY(t *testing.T) {
+	// When stdin is not a terminal (e.g. in CI), ReadPassword should
+	// return an error rather than hanging or panicking.
+	_, err := ReadPassword("Password: ")
+	if err == nil {
+		t.Fatal("expected error when stdin is not a terminal")
+	}
+}