feat(diary): boot --help invocations with the console at warn (quick-261028-rqc)

- Options.Args holds the command-line arguments; FromGlobals fills it
  from os.Args[1:], so the generated main needs no change
- Bootstrap and Setup install the setup with Quiet when Args only ask
  for help (-h, --help) and no -v is given; files keep their levels
- TestBootQuietSmoke runs the generated main's sequence with a plugin
  that logs from Register and Boot

modules/diary/helponly.go

@@ -0,0 +1,55 @@
+package diary
+
+// helpOnly reports whether args, a binary's command line without the
+// program name, only ask for help: before "--" they hold --help or a
+// single-dash cluster that reaches h. It is a tolerant pre-scan of the raw
+// arguments, made before plugins boot, and reads them the way
+// bonfire.PeekGlobals does: it stops at "--", skips the value of --env and
+// reads a single-dash cluster letter by letter, stopping at the first
+// letter that is not a global shorthand (v, q, n, h). It never fails and
+// cannot tell a passthrough command (whose --help reaches its Run) from
+// another, so "<passthrough> --help" counts; the root's OnGlobals hook
+// (ApplyGlobals) restores the console levels before that command runs.
+func helpOnly(args []string) bool {
+	for i := 0; i < len(args); i++ {
+		a := args[i]
+		switch {
+		case a == "--":
+			return false
+		case a == "--env":
+			i++
+		case a == "--help":
+			return true
+		case len(a) >= 2 && a[0] == '-' && a[1] != '-':
+			if clusterHasHelp(a[1:]) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// clusterHasHelp reports whether a shorthand cluster (the letters after the
+// dash) reaches h before a letter that is not a global shorthand.
+func clusterHasHelp(cluster string) bool {
+	for i := 0; i < len(cluster); i++ {
+		switch cluster[i] {
+		case 'h':
+			return true
+		case 'v', 'q', 'n':
+		default:
+			return false
+		}
+	}
+	return false
+}
+
+// bootQuiet applies the help-only rule of Bootstrap and Setup: with Args
+// set, no -v and arguments that only ask for help, the setup is installed
+// with Quiet, which raises only the console channels to warn.
+func bootQuiet(opts Options) Options {
+	if opts.Args != nil && opts.Verbosity == 0 && helpOnly(opts.Args) {
+		opts.Quiet = true
+	}
+	return opts
+}