Adds TaskField service implementing tasks.task.field.get and tasks.task.field.list REST API v3 methods.

New files

• src/Services/Task/TaskField/Service/TaskField.php — service with get(string $name, array $select = []) and list(array $select = []) methods
• src/Services/Task/TaskField/Result/TaskFieldItemResult.php — field descriptor item with @property-read annotations for all 11 fields
• src/Services/Task/TaskField/Result/TaskFieldResult.php — single-item result (reads from result['item'])
• src/Services/Task/TaskField/Result/TaskFieldsResult.php — list result (reads from result['items'])

Modified files

• src/Services/Task/TaskServiceBuilder.php — added taskField() accessor
• phpunit.xml.dist — added integration_tests_task_field suite
• Makefile — added test-integration-task-field target
• CHANGELOG.md — added entry under ### Added

Usage

$taskField = $serviceBuilder->getTaskScope()->taskField();

// get single field descriptor
$field = $taskField->get('id')->taskField();
echo $field->name;  // 'id'
echo $field->type;  // 'int'

// list all field descriptors
$fields = $taskField->list()->getTaskFields();

Test plan

• make lint-cs-fixer — passed
• make lint-rector — passed (variable renamed to match return type)
• make lint-phpstan — passed
• make lint-deptrac — passed (0 violations)
• make test-unit — passed (653 tests)
• make test-integration-task-field — passed (5 tests, 16 assertions)

Closes #395

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