PR Core v0.5.0: Add Repo Daily CPT, taxonomy, and metaboxes

- Register repo_daily CPT at /daily/[slug]/ for editorial publications
- Register repo_daily_category taxonomy with 4 seeded terms
- Add Repo Daily metabox for author byline and clinical review flag
- Bulk-migrate _repo_daily_author on autoblogged posts (psa_source)
- Update ARCHITECTURE.md with content-type table and scanner scope
- Update CONVENTIONS.md with repo_daily meta field extension pattern
- Bump version to 0.5.0, add CHANGELOG entry

Closes v0.5.0 scope.

includes/taxonomies/class-pr-core-repo-daily-taxonomy.php

@@ -0,0 +1,78 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * Registers the repo_daily_category taxonomy for Repo Daily articles.
+ *
+ * What: Defines the `repo_daily_category` taxonomy attached to repo_daily CPT.
+ *       Seeds four content categories on activation: article, guide, comparison, news.
+ * Who calls it: PR_Core::init() on plugins_loaded; activation hook seeding.
+ * Dependencies: None.
+ *
+ * Non-hierarchical taxonomy with REST support. Used by editorial and publishing
+ * workflows to categorize Repo Daily content.
+ *
+ * @see ARCHITECTURE.md — Content type taxonomy specification.
+ * @see CONVENTIONS.md — Taxonomy ownership pattern.
+ */
+class PR_Core_Repo_Daily_Taxonomy {
+
+	/** @var string Taxonomy: repo_daily_category. */
+	public const TAX_CATEGORY = 'repo_daily_category';
+
+	/**
+	 * Register WordPress hooks for taxonomy and seeding.
+	 *
+	 * @return void
+	 */
+	public function register_hooks(): void {
+		add_action( 'init', [ __CLASS__, 'register_taxonomy' ] );
+	}
+
+	/**
+	 * Register the `repo_daily_category` taxonomy.
+	 *
+	 * Side effects: registers taxonomy with WordPress.
+	 *
+	 * @return void
+	 */
+	public static function register_taxonomy(): void {
+		if ( taxonomy_exists( self::TAX_CATEGORY ) ) {
+			return;
+		}
+
+		register_taxonomy( self::TAX_CATEGORY, PR_Core_Repo_Daily_CPT::POST_TYPE, [
+			'labels'             => [
+				'name'          => __( 'Category', 'peptide-repo-core' ),
+				'singular_name' => __( 'Category', 'peptide-repo-core' ),
+			],
+			'public'             => true,
+			'publicly_queryable' => true,
+			'show_in_rest'       => true,
+			'show_ui'            => true,
+			'show_admin_column'  => true,
+			'hierarchical'       => false,
+			'rewrite'            => [ 'slug' => 'daily/category', 'with_front' => false ],
+		] );
+	}
+
+	/**
+	 * Seed default category terms on activation.
+	 *
+	 * Idempotent: checks for existence of each term before inserting.
+	 * Called by PR_Core_Activator during plugin activation.
+	 *
+	 * Side effects: may insert terms into wp_terms and wp_term_taxonomy.
+	 *
+	 * @return void
+	 */
+	public static function seed_terms(): void {
+		$terms = [ 'article', 'guide', 'comparison', 'news' ];
+
+		foreach ( $terms as $term ) {
+			if ( ! term_exists( $term, self::TAX_CATEGORY ) ) {
+				wp_insert_term( $term, self::TAX_CATEGORY );
+			}
+		}
+	}
+}