style(setup): address actionable golangci-lint findings

Fixes the lint hits that pointed at real bugs or dead code; leaves the
remaining style-only suggestions (rangeValCopy micro-copies, gocyclo
informational, intentional name choices like SetupStateMachine) alone.

- pkg/models/clockdisplay.go: restore <clockDisplay> XMLName tag on both
  ClockDisplay and ClockDisplayRequest. The earlier `xml:"-"` clashed
  with ClockDisplayUpdatedEvent.ClockDisplay's `xml:"clockDisplay"` tag
  (SA5008). Custom MarshalXML/UnmarshalXML still own the wire format.
- pkg/service/setup/setup.go: drop the now-unused checkRemoteServices
  helper (replaced by applyProbeToSummary) and rename the unused
  deviceIP parameter of populatePlannedNetworkConfig to _.
- pkg/service/setup/setup_session.go: collapse sendStep's (string, error)
  return to plain error — every caller already discarded the string.
- pkg/service/setup/init_plan.go: rename shadowed err variables to
  rwErr / genErr / invalidErr / nilErr / stepErr.
- cmd/soundtouch-cli/cmd_setup.go: drop redundant int(syscall.Stdin)
  conversion (already int) and rename a shadowed err to pairErr.

go build ./..., go vet ./..., and tests for the touched packages all
green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pkg/models/clockdisplay.go

@@ -28,7 +28,7 @@ import (
 // other callers don't have to be rewritten; custom MarshalXML /
 // UnmarshalXML methods bridge to the nested format on the wire.
 type ClockDisplay struct {
-	XMLName    xml.Name `xml:"-"`
+	XMLName    xml.Name `xml:"clockDisplay"`
 	DeviceID   string
 	Enabled    bool
 	Format     string // public-facing values: "12", "24", "auto"
@@ -234,7 +234,7 @@ func (c *ClockDisplay) IsEmpty() bool {
 // settings. Fields use the same public names as the response struct;
 // MarshalXML produces the nested wire format the device requires.
 type ClockDisplayRequest struct {
-	XMLName    xml.Name `xml:"-"`
+	XMLName    xml.Name `xml:"clockDisplay"`
 	Enabled    *bool
 	Format     string
 	Brightness *int