fix: verify NKey file has secure permissions before reading

Reject NKey files that are readable by group or others (permissions
more permissive than 0600). This prevents accidental exposure of
private keys through overly permissive file permissions.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/homelab-deploy/main.go

@@ -16,7 +16,7 @@ import (
 	"github.com/urfave/cli/v3"
 )
 
-const version = "0.1.0"
+const version = "0.1.1"
 
 func main() {
 	app := &cli.Command{