feat(v0.5.3): doctor -- checkTmux three-state helper

cmd/doctor.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/spf13/cobra"
 	"github.com/warrenronsiek/ctask/internal/config"
+	"github.com/warrenronsiek/ctask/internal/shell"
 	"github.com/warrenronsiek/ctask/internal/workspace"
 )
 
@@ -203,6 +204,9 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 	// Check 8: CTASK_PROJECT_ROOT (v0.5).
 	checkProjectRoot(&passed, &failed)
 
+	// Check 9: tmux availability for persistent session mode (v0.5.3).
+	checkTmux(&passed, &failed)
+
 	// Summary
 	fmt.Println()
 	fmt.Printf("%d checks passed, %d failed\n", passed, failed)
@@ -254,3 +258,29 @@ func checkSeedDir(label, envValue, resolved, envName string, passed, failed *int
 	fmt.Printf("         Fix: create the directory or unset %s to use built-in defaults.\n", envName)
 	*failed++
 }
+
+// checkTmux reports the three-state tmux check (v0.5.3):
+//   - CTASK_SESSION_MODE != "persistent" -> INFO (direct mode, tmux optional)
+//   - persistent + tmux on PATH + version OK -> two INFO lines
+//   - persistent + tmux missing or too old -> FAIL with install/update hint
+func checkTmux(passed, failed *int) {
+	_ = passed
+	mode := config.ResolveSessionMode()
+	if mode != "persistent" {
+		fmt.Printf("  [INFO] Session mode: direct (tmux not required)\n")
+		return
+	}
+	fmt.Printf("  [INFO] Session mode: persistent\n")
+	tmuxPath, ver, err := shell.LookupTmux()
+	if err != nil {
+		fmt.Printf("  [FAIL] tmux not found on PATH or unsupported version: %v\n", err)
+		fmt.Printf("         Fix: install tmux 3.0+ (apt/brew/pacman/dnf), or unset CTASK_SESSION_MODE\n")
+		*failed++
+		return
+	}
+	rawVer := ver.Raw
+	if rawVer == "" {
+		rawVer = "unknown version"
+	}
+	fmt.Printf("  [INFO] tmux found: %s (%s)\n", rawVer, tmuxPath)
+}