President Donald Trump’s proposed “ Golden Dome ” missile defense system could cost as much as $1.2 trillion over two decades, according to a new estimate from the Congressional Budget Office, far exceeding the $175 billion price tag he originally offered. The nonpartisan budget office cautioned that its analysis was not based on a finalized blueprint because the Defense Department has not publicly detailed the full architecture of a complex system that Trump has touted as “forever ending the missile threat to the American homeland.” Instead, the office said its estimate reflected “one illustrative approach rather than an estimate of a specific Administration proposal.” The report offered the clearest indication yet of how expensive the ambitious project could become, as well as the enormous technical and financial challenges of building a space-age shield against missile attacks.…