feat(cost): persist run_id on generated posts + back-populate for v0.8.1 reaper

The orphan-research-row reaper (upcoming commit) needs a cheap way to
answer "which posts belong to this run_id?" when amortizing a stranded
`llm_research` row back to its siblings. Relying on the generation_log
post_id column is circular: if the pipeline died before amortize_research
ran, we'd trust the same table we're trying to repair.

- `Publisher::build_meta()` now accepts `?string $run_id` and writes
  `_prautoblogger_run_id => $run_id` when non-empty. Null/empty preserves
  existing null-safe legacy behaviour.
- `Publisher::create_post()` forwards the $run_id it already receives
  into build_meta — one-line plumbing, no new parameters at call sites.
- `Activator::backpopulate_run_id_meta_v081()` runs once per site,
  gated by `prautoblogger_migrated_run_id_meta_v081` option. Walks
  `wp_prautoblogger_generation_log`, pairs (post_id, min(run_id)),
  writes `_prautoblogger_run_id` on any post that doesn't already have
  it. Uses `metadata_exists()` so it's a no-op on re-run.
- Activator also schedules the new daily `prautoblogger_reap_orphan_research_rows`
  cron at 03:15 — 15 min after the primary generation cron — so live
  pipelines have time to finish amortizing cleanly before the reaper scans.
- Deactivator unschedules the new hook alongside the existing cron hooks.

No user-visible behaviour change yet. The reaper handler ships in the
next commit; this one just ensures it will have the data + schedule it
needs when it lands.

Part of the v0.8.1 bundle (thread research-cost-reaper).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

includes/core/class-publisher.php

@@ -87,7 +87,7 @@ class PRAutoBlogger_Publisher {
 			'post_status'  => $post_status,
 			'post_type'    => 'post',
 			'post_author'  => PRAutoBlogger_Post_Assembler::get_default_author_id(),
-			'meta_input'   => $this->build_meta( $idea, $review ),
+			'meta_input'   => $this->build_meta( $idea, $review, $run_id ),
 		];
 
 		/** @see class-prautoblogger.php — listeners registered in main loader. */
@@ -120,15 +120,22 @@ class PRAutoBlogger_Publisher {
 	/**
 	 * Build post_meta array for generation metadata.
 	 *
+	 * The `_prautoblogger_run_id` key (added v0.8.1) lets the orphan-research
+	 * reaper attribute an orphan `llm_research` row back to its sibling
+	 * articles without re-walking the gen_log table. See
+	 * core/class-research-reaper.php.
+	 *
 	 * @param PRAutoBlogger_Article_Idea     $idea
 	 * @param PRAutoBlogger_Editorial_Review $review
+	 * @param string|null                    $run_id Pipeline run UUID, or null in legacy paths.
 	 * @return array<string, mixed>
 	 */
 	private function build_meta(
 		PRAutoBlogger_Article_Idea $idea,
-		PRAutoBlogger_Editorial_Review $review
+		PRAutoBlogger_Editorial_Review $review,
+		?string $run_id = null
 	): array {
-		return [
+		$meta = [
 			'_prautoblogger_generated'       => '1',
 			'_prautoblogger_analysis_id'     => $idea->get_analysis_id(),
 			'_prautoblogger_source_ids'      => wp_json_encode( $idea->get_source_ids() ),
@@ -143,5 +150,9 @@ class PRAutoBlogger_Publisher {
 			'_prautoblogger_article_type'    => $idea->get_article_type(),
 			'_prautoblogger_target_keywords' => wp_json_encode( $idea->get_target_keywords() ),
 		];
+		if ( null !== $run_id && '' !== $run_id ) {
+			$meta['_prautoblogger_run_id'] = $run_id;
+		}
+		return $meta;
 	}
 }