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>

i3bar.go

@@ -1,8 +1,14 @@
 package main
 
 import (
+	"bufio"
+	"encoding/json"
 	"fmt"
+	"log"
+	"os"
+	"os/exec"
 	"strings"
+	"time"
 )
 
 // I3BarBlock represents a single block in the i3bar protocol.
@@ -50,3 +56,121 @@ func formatI3BarBlocks(sessions []SessionInfo) []I3BarBlock {
 
 	return []I3BarBlock{{FullText: text, Color: color, Name: "vmux"}}
 }
+
+// queryVmuxBlock fetches sessions from daemon and returns the vmux i3bar block.
+// Returns a "daemon offline" block if the daemon is unreachable.
+func queryVmuxBlock(sockPath string) I3BarBlock {
+	client := NewClient(sockPath)
+	resp, err := client.Send(Request{Action: "list"})
+	if err != nil {
+		return I3BarBlock{FullText: "vmux: daemon offline", Color: "#888888", Name: "vmux"}
+	}
+	if !resp.OK {
+		return I3BarBlock{FullText: "vmux: error", Color: "#888888", Name: "vmux"}
+	}
+	blocks := formatI3BarBlocks(resp.Sessions)
+	return blocks[0]
+}
+
+// writeI3BarLine writes a JSON array of blocks as an i3bar protocol line.
+// Uses os.Stdout.Write + Sync to avoid buffering issues.
+func writeI3BarLine(blocks []I3BarBlock, first bool) {
+	data, err := json.Marshal(blocks)
+	if err != nil {
+		log.Printf("i3bar marshal error: %v", err)
+		return
+	}
+
+	var line string
+	if first {
+		line = string(data) + "\n"
+	} else {
+		line = "," + string(data) + "\n"
+	}
+	os.Stdout.Write([]byte(line))
+	os.Stdout.Sync()
+}
+
+// runI3Bar runs the i3bar protocol loop on stdout.
+// If i3statusCmd is non-empty, it wraps i3status output and prepends vmux blocks.
+// If empty, runs in standalone mode with periodic polling.
+func runI3Bar(sockPath, i3statusCmd string) {
+	if i3statusCmd != "" {
+		runI3BarWrapped(sockPath, i3statusCmd)
+		// If i3status exits, fall through to standalone
+	}
+	runI3BarStandalone(sockPath)
+}
+
+func runI3BarStandalone(sockPath string) {
+	os.Stdout.Write([]byte("{\"version\":1}\n"))
+	os.Stdout.Write([]byte("[\n"))
+	os.Stdout.Sync()
+
+	first := true
+	for {
+		block := queryVmuxBlock(sockPath)
+		writeI3BarLine([]I3BarBlock{block}, first)
+		first = false
+		time.Sleep(2 * time.Second)
+	}
+}
+
+func runI3BarWrapped(sockPath, i3statusCmd string) {
+	cmd := exec.Command("sh", "-c", i3statusCmd)
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		log.Printf("i3bar: cannot pipe i3status stdout: %v", err)
+		return
+	}
+	cmd.Stderr = os.Stderr
+
+	if err := cmd.Start(); err != nil {
+		log.Printf("i3bar: cannot start i3status: %v", err)
+		return
+	}
+
+	scanner := bufio.NewScanner(stdout)
+	scanner.Buffer(make([]byte, 0, 64*1024), 1024*1024)
+
+	// Forward header line (e.g. {"version":1})
+	if !scanner.Scan() {
+		log.Print("i3bar: i3status produced no output")
+		return
+	}
+	os.Stdout.Write([]byte(scanner.Text() + "\n"))
+	os.Stdout.Sync()
+
+	// Forward opening bracket
+	if !scanner.Scan() {
+		log.Print("i3bar: i3status produced no opening bracket")
+		return
+	}
+	os.Stdout.Write([]byte(scanner.Text() + "\n"))
+	os.Stdout.Sync()
+
+	first := true
+	for scanner.Scan() {
+		line := scanner.Text()
+		trimmed := strings.TrimLeft(line, ",")
+		if trimmed == "" {
+			continue
+		}
+
+		var blocks []I3BarBlock
+		if err := json.Unmarshal([]byte(trimmed), &blocks); err != nil {
+			// Not a JSON array line, forward as-is
+			os.Stdout.Write([]byte(line + "\n"))
+			os.Stdout.Sync()
+			continue
+		}
+
+		vmuxBlock := queryVmuxBlock(sockPath)
+		blocks = append([]I3BarBlock{vmuxBlock}, blocks...)
+
+		writeI3BarLine(blocks, first)
+		first = false
+	}
+
+	cmd.Wait()
+}