Add opt-in checkpoints to operations

[stevebauman]

Long-running operations currently have no built-in way to save progress between attempts. I've added an opt-in DirectoryTree\Operations\Concerns\HasCheckpoints trait so an operation can persist a cursor or other JSON value and read it when it runs again:

$lastId = $this->checkpoints->get('last_id', 0);

$this->checkpoints->put('last_id', $company->id);

get() reads a value with an optional default. put() saves a key and value, or an array of values, without replacing other keys. Checkpoints are scoped to the operation's filename, use the configured operations connection, and participate in its active transaction.

Applications that use HasCheckpoints can publish and run its migration separately:

php artisan vendor:publish --tag=operations-checkpoints-migration
php artisan migrate

The standard migration tag only publishes the operations table. Running, checking status, and forgetting operations work without the checkpoint table. When it exists, operations:forget also clears saved checkpoints, including progress from unfinished operations. The repository's forget() method clears only the operation's checkpoints.

Validation: 72 tests pass (336 assertions), and Pint passes locally.