add assignments name=value and unset

- name=value (one or more) without a command sets variables of the shell
- name=value before a command sets them only in its environment (for
  builtins only while they run)
- an assignment needs an unquoted name and =; the value is not split
  and not expanded as wildcard or brace pattern, ~ is expanded after =
  and :
- the status of an assignment is the one of its last $(...)
- new builtin unset

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

brace.go

@@ -22,6 +22,7 @@ func expandBraceWords(input string) (string, error) {
 	var (
 		out           strings.Builder
 		afterRedirect bool // the next word is the target of a redirection
+		commandStart  = true
 	)
 
 	for i := 0; i < len(input); {
@@ -33,6 +34,8 @@ func expandBraceWords(input string) (string, error) {
 		case strings.IndexByte(";&|<>", ch) >= 0:
 			if ch == '<' || ch == '>' {
 				afterRedirect = true
+			} else {
+				commandStart = true
 			}
 			out.WriteByte(ch)
 			i++
@@ -42,6 +45,16 @@ func expandBraceWords(input string) (string, error) {
 			return out.String(), nil
 		default:
 			end := wordEnd(input, i)
+			if commandStart && !afterRedirect && isAssignment(input[i:end]) {
+				// name=value before the command: no brace expansion, like
+				// in bash
+				out.WriteString(input[i:end])
+				i = end
+				continue
+			}
+			if !afterRedirect {
+				commandStart = false
+			}
 			words, err := expandBraces(input[i:end])
 			if err != nil {
 				return "", err
@@ -57,6 +70,12 @@ func expandBraceWords(input string) (string, error) {
 	return out.String(), nil
 }
 
+// isAssignment reports whether a word starts with name= (unquoted).
+func isAssignment(word string) bool {
+	name, _, ok := strings.Cut(word, "=")
+	return ok && isVarName(name)
+}
+
 // wordEnd returns the end of the word starting at i: the next unquoted blank
 // or operator character.
 func wordEnd(s string, i int) int {