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.go

@@ -0,0 +1,22 @@
+// Package terminal provides secure terminal input helpers for CLI tools.
+package terminal
+
+import (
+	"fmt"
+	"os"
+
+	"golang.org/x/term"
+)
+
+// ReadPassword prints the given prompt to stderr and reads a password
+// from the terminal with echo disabled. It prints a newline after the
+// input is complete so the cursor advances normally.
+func ReadPassword(prompt string) (string, error) {
+	fmt.Fprint(os.Stderr, prompt)
+	b, err := term.ReadPassword(int(os.Stdin.Fd())) //nolint:gosec // fd fits in int
+	fmt.Fprintln(os.Stderr)
+	if err != nil {
+		return "", err
+	}
+	return string(b), nil
+}

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")
+	}
+}