Adopt reusable CI standard (#30)

Squash-merged from claude/adopt-ci-20260616.

QA-approved; all CI checks green on 3b4e3f04.

Agent-Session: deploy-ci-rollout-news-plugin-2026-06-16

includes/class-peptide-news-rest-api.php

@@ -104,6 +104,7 @@ class Peptide_News_Rest_API {
 		if ( ! preg_match( '/^\\d{4}-\\d{2}-\\d{2}$/', $value ) ) {
 			return new WP_Error(
 				'rest_invalid_date',
+				/* translators: %s: REST API parameter name */
 				sprintf( __( 'Invalid date format for %s. Expected Y-m-d.', 'peptide-news' ), $param ),
 				array( 'status' => 400 )
 			);