fix: Add Windows build support for daemon command

- Split platform-specific daemon process configuration into separate files
- daemon_unix.go: Uses Setsid for Unix/Linux/macOS
- daemon_windows.go: Uses CREATE_NEW_PROCESS_GROUP for Windows
- Fixes compilation error: "unknown field Setsid in struct literal"

This allows bd.exe to build successfully on Windows while maintaining
proper daemon behavior on all platforms.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/daemon.go

@@ -262,7 +262,7 @@ func startDaemon(interval time.Duration, autoCommit, autoPush bool, logFile, pid
 
 	cmd := exec.Command(exe, args...)
 	cmd.Env = append(os.Environ(), "BD_DAEMON_FOREGROUND=1")
-	cmd.SysProcAttr = &syscall.SysProcAttr{Setsid: true}
+	configureDaemonProcess(cmd)
 	
 	devNull, err := os.OpenFile(os.DevNull, os.O_RDWR, 0)
 	if err != nil {