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/color.go

@@ -3,6 +3,7 @@
 package main
 
 import (
+	"encoding/json"
 	"strings"
 
 	cli "git.ipng.ch/ipng/golang-cli"
@@ -10,10 +11,10 @@ import (
 
 // formatError returns a user-friendly error string. gRPC status errors are
 // unwrapped to show only the server's message (no "rpc error: code = ..."
-// boilerplate). The result is wrapped in red when color is enabled. The label()
-// helper and the ANSI palette now live in the golang-cli library (cli.Label,
-// cli.Red, ...); only this gRPC-specific unwrap stays here, wired in as
-// App.FormatError.
+// boilerplate). In JSON mode it returns the message as a {"error": "..."}
+// document (so failures are machine-readable, matching the data/`{}` a success
+// prints); otherwise it wraps the message in red when color is enabled.
+// App.FormatError prints the result.
 func formatError(err error) string {
 	msg := err.Error()
 	// google.golang.org/grpc/status errors format as:
@@ -21,6 +22,10 @@ func formatError(err error) string {
 	if i := strings.Index(msg, " desc = "); i >= 0 {
 		msg = msg[i+len(" desc = "):]
 	}
+	if cli.IsJSON() {
+		b, _ := json.Marshal(map[string]string{"error": msg})
+		return string(b)
+	}
 	if cli.ColorEnabled() {
 		return cli.Red + msg + cli.Reset
 	}