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>

go.sum

@@ -1,5 +1,7 @@
 git.ipng.ch/ipng/golang-cli v1.3.0 h1:E26W55czJSl+kmSXpwFWxYHsnaT84unsNBtfdM3iT4M=
 git.ipng.ch/ipng/golang-cli v1.3.0/go.mod h1:8yeV4X7MF5hKQnxnYYJKyqby4P58EtH82zd36BrNbqY=
+git.ipng.ch/ipng/golang-cli v1.4.0 h1:KpvVqkieYoH3en7ay0QGmagiYcXJoUAiU49OLx3eZGw=
+git.ipng.ch/ipng/golang-cli v1.4.0/go.mod h1:8yeV4X7MF5hKQnxnYYJKyqby4P58EtH82zd36BrNbqY=
 github.com/aymanbagabas/go-osc52/v2 v2.0.1 h1:HwpRHbFMcZLEVr42D4p7XBqjyuxQH5SMiErDT4WkJ2k=
 github.com/aymanbagabas/go-osc52/v2 v2.0.1/go.mod h1:uYgXzlJ7ZpABp8OJ+exZzJJhRNQ2ASbcXHWsFqH8hp8=
 github.com/beorn7/perks v1.0.1 h1:VlbKKnNfV8bJzeqoa4cOKqO6bYr3WgKZxO8Z16+hsOM=