Print banner when vaultik is invoked with no subcommand

Cobra's default 'no subcommand → print help' path bypasses fx, so
the startup banner never ran for bare 'vaultik'. Add a Run handler
on the root command that prints the banner and then calls Help.

Extracted the banner-printing logic into writeStartupBanner() so
both this path and the fx setupGlobals hook share one implementation.

internal/cli/root.go

@@ -5,9 +5,12 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"time"
 
 	"github.com/adrg/xdg"
 	"github.com/spf13/cobra"
+	"sneak.berlin/go/vaultik/internal/globals"
+	"sneak.berlin/go/vaultik/internal/ui"
 )
 
 // RootFlags holds global flags that apply to all commands.
@@ -32,6 +35,18 @@ func NewRootCommand() *cobra.Command {
 public keys and uploads to S3-compatible storage. No private keys are needed
 on the source system.`,
 		SilenceUsage: true,
+		// When invoked with no subcommand, print the banner then the
+		// usage/help. Cobra's default behavior (without a Run) just
+		// prints help, which skips the banner.
+		Run: func(cmd *cobra.Command, args []string) {
+			startTime := time.Now().UTC()
+			short := globals.Commit
+			if len(short) > 12 {
+				short = short[:12]
+			}
+			writeStartupBanner(ui.New(os.Stdout), startTime, short)
+			_ = cmd.Help()
+		},
 	}
 
 	// Add global flags