Tell harness transient experiment

<?php
require 'examples/boot.php';
require_once dirname(__DIR__).'/Support.php';

use Cognesy\Tell\Tell;
use Cognesy\Tell\TellRequest;

$project = TellHarnessExample::project();

try {
    $tell = Tell::open($project);
    $tell->workspace()->initialize();
    $review = $tell->conversation('release-review');

    $review->send(TellRequest::prompt('Record the current release decision.'));
    $before = $review->history()->head;

    $prompt = 'Challenge that decision and describe the strongest alternative.';
    $experiment = $tell->run(
        TellRequest::prompt($prompt)
            ->conversation('release-review')
            ->transient(),
    );
    $after = $review->history()->head;

    echo "=== Transient Result ===\n";
    echo $experiment->text(), "\n";
    echo 'Transient: '.($experiment->isTransient() ? 'yes' : 'no')."\n";
    echo 'Conversation head unchanged: '.($before === $after ? 'yes' : 'no')
        ."\n";

    assert($experiment->isTransient(), 'Expected a transient result.');
    assert(
        $before === $after,
        'A transient run must not move the conversation head.',
    );
} finally {
    TellHarnessExample::remove($project);
}