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/class-deactivator.php

@@ -36,6 +36,7 @@ class PRAutoBlogger_Deactivator {
 		$hooks = [
 			'prautoblogger_daily_generation',
 			'prautoblogger_collect_metrics',
+			'prautoblogger_reap_orphan_research_rows',
 		];
 
 		foreach ( $hooks as $hook ) {