feat(04-02): add vmux i3bar subcommand with i3status wrapping

- runI3Bar: standalone mode (polling 2s) and i3status wrapping mode
- i3bar protocol v1: header + infinite JSON array on stdout
- queryVmuxBlock: daemon offline fallback in gray
- CLI: vmux i3bar auto-detects i3status in PATH
- Stdout sync after each write to avoid buffering

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

main.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"strconv"
 	"strings"
@@ -141,6 +142,17 @@ func main() {
 		}
 		fmt.Println("Daemon stopped.")
 
+	case "i3bar":
+		if err := EnsureDaemon(sockPath); err != nil {
+			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+			os.Exit(1)
+		}
+		i3statusCmd := ""
+		if _, err := exec.LookPath("i3status"); err == nil {
+			i3statusCmd = "i3status"
+		}
+		runI3Bar(sockPath, i3statusCmd)
+
 	case "daemon":
 		runDaemon(sockPath)
 
@@ -199,6 +211,7 @@ Commands:
   switch <query>    Switch to the workspace of the matching session
   label <id> <text> Assign a label to a session
   focus <minutes>   Suppress notifications for N minutes
+  i3bar             Output i3bar JSON (use as status_command in i3 config)
   stop              Stop the vmux daemon
   daemon            Run the daemon in foreground (used internally)