cli: simplify option model per pike review

- appendList: one home for the List newline-join rule (was duplicated
  across accumulate and set)
- fold the redundant Int64 getter into Int; drop the truthy helper for
  the shared boolWord vocabulary
- add Opt.Metavar so help derives the value placeholder from data
  instead of branching on the option name (seed-time)

No behavior change; --help output is byte-identical.

cli/options.go

@@ -48,13 +48,11 @@ func (o *Options) Bool(name string) bool {
 	return val
 }
 
-// Int returns the value as an int, or 0 if empty/invalid.
-func (o *Options) Int(name string) int { return int(o.Int64(name)) }
-
-// Int64 returns the value as an int64, or 0 if empty/invalid.
-func (o *Options) Int64(name string) int64 {
+// Int returns the value as an int, or 0 if empty/invalid. The value has already
+// passed validate(), so a parse failure here is unreachable in normal flow.
+func (o *Options) Int(name string) int {
 	n, _ := strconv.ParseInt(strings.TrimSpace(o.vals[name]), 10, 64)
-	return n
+	return int(n)
 }
 
 // Float returns the value as a float64, or 0 if empty/invalid.