feat(maglevc): JSON output for all commands (-json)

Enable App.JSON and add -json on golang-cli v1.4.0, mirroring evpnc so
the two CLIs' JSON contract is identical: show/query -> data, set/action
-> {}, failure -> {"error": "..."}.

- show/query commands branch on cli.IsJSON() -> emit the protobuf via
  cli.EmitJSON; text keeps the tabwriter painters (which robot tests
  parse via show, not via setter output).
- action commands (pause/resume/enable/disable, set weight, sync,
  config reload) are now silent on success in text too — "we did what
  you asked" needs no confirmation — and print "{}" in JSON via wrapJSON.
- config check stays informative (it is a query): text "config ok" /
  error, JSON the CheckConfig report.
- errors: formatError returns {"error": "..."} in JSON mode.
- watch streams its own JSON events (no trailing {}).

Robot tests assert backend state via `show`, not setter stdout, so the
dropped confirmations don't affect them. Builds on linux and openbsd.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/client/watch.go

@@ -26,7 +26,8 @@ func dynWatchEventOpts(_ context.Context, _ grpcapi.MaglevClient, _ []string) []
 // All tokens after 'events' are captured as args by the circular slot node in the tree.
 // If none of log/backend/frontend are mentioned, all three default to true.
 func runWatchEvents(ctx context.Context, client grpcapi.MaglevClient, args []string) error {
-	var maxEvents int // 0 = unlimited
+	jsonEmitted = true // watch streams its own JSON event lines; never append "{}"
+	var maxEvents int  // 0 = unlimited
 	var wantLog, wantBackend, wantFrontend bool
 	logLevel := ""
 	anyExplicit := false