chore(update): tidy what the removed Windows helper left behind

The no-op main builds on every platform but Linux so Windows no longer has an
empty package, fixes its typo, and drops two comments that still described the
Windows helper.

desktop/cmd/update-helper/main_other.go

@@ -1,4 +1,4 @@
-//go:build !windows && !linux
+//go:build !linux
 
 package main
 
@@ -8,6 +8,6 @@ import (
 )
 
 func main() {
-	fmt.Fprintln(os.Stderr, "reasonix-update-helper is is only used by Linux builds")
+	fmt.Fprintln(os.Stderr, "reasonix-update-helper is only used by Linux builds")
 	os.Exit(2)
 }

internal/platform/repair/apply_failure.go

@@ -13,10 +13,7 @@ import (
 )
 
 // UpdateApplyFailure records that an update installer failed after the desktop
-// handed off and exited. The Windows update helper cannot roll back itself —
-// it runs from the cache directory, outside the validated Guard installation —
-// so it records this marker and relaunches Guard, which performs the rollback
-// from inside the install directory on its next start.
+// handed off and exited.
 type UpdateApplyFailure struct {
 	SchemaVersion       int    `json:"schemaVersion"`
 	ToVersion           string `json:"toVersion,omitempty"`

internal/platform/update/apply.go

@@ -11,7 +11,7 @@ import (
 // directory, not undoing a newer one — so nothing here asks the direction.
 type VersionedInstaller struct {
 	Layout  Layout
-	Staging string // update cache dir; the Windows helper copy is placed here
+	Staging string // update cache dir
 	Current string // running version, recorded in the macOS handoff transaction
 	Line    Line   // which product line's members this archive carries
 	App     Application