Fix compilation errors in linting improvements

- Remove duplicate emptyString constant declaration
- Fix variable assignment in Linux posture checker
- Ensure builds pass with linting improvements intact

Co-authored-by: Amp <amp@ampcode.com>
Amp-Thread-ID: https://ampcode.com/threads/T-5be4213f-26eb-400c-bb7b-d4c79b7ee6fe

Author: haasonsaas

agent/internal/posture/linux.go

@@ -176,7 +176,7 @@ func (c *LinuxCollector) checkSystemUpdated() bool {
 	}
 
 	// Try yum/dnf for Red Hat systems
-	output, err := runCommand("yum", "check-update")
+	output, err = runCommand("yum", "check-update")
 	if err != nil {
 		// Exit code 0 means no updates, 100 means updates available
 		return true

pkg/secrets/secrets.go

@@ -7,10 +7,6 @@ import (
 	"strings"
 )
 
-const (
-	emptyString = ""
-)
-
 // Manager defines the interface for secret management
 type Manager interface {
 	GetSecret(ctx context.Context, key string) (string, error)