feat: migrate GitHubIssueTask + WorktreeCleanupTask to executeTask() contract

Renames execute() to executeTask() in both task classes to match the
updated SystemTask abstract contract from data-machine 0.72.0.

Part of coordinated big-bang migration (data-machine#1153).

Author: chubes4

inc/Tasks/GitHubIssueTask.php

@@ -27,7 +27,7 @@ class GitHubIssueTask extends SystemTask {
 	 * @param int   $jobId  Job ID from DM Jobs table.
 	 * @param array $params Task parameters from engine_data.
 	 */
-	public function execute( int $jobId, array $params ): void {
+	public function executeTask( int $jobId, array $params ): void {
 		$result = GitHubAbilities::createIssue( $params );
 
 		if ( is_wp_error( $result ) ) {

inc/Tasks/WorktreeCleanupTask.php

@@ -117,7 +117,7 @@ public static function getTaskMeta(): array {
 	 * @param int   $jobId  Job ID from DM Jobs table.
 	 * @param array $params Task parameters from engine_data.
 	 */
-	public function execute( int $jobId, array $params ): void {
+	public function executeTask( int $jobId, array $params ): void {
 		// Gate on the same PluginSetting the React UI toggles and the
 		// recurring schedule reads. Defensive — scheduled runs already
 		// wouldn't fire when the setting is off, but manual runs via