feat(cost): daily reaper for orphan llm_research rows

Implements the orphan-research-row reaper per thread research-cost-reaper
seq 1. When a pipeline dies between post creation and
`Post_Assembler::amortize_research_costs()` (Hostinger exec kill, OOM,
LiteSpeed cut, fatal), the shared research row stays in
`wp_prautoblogger_generation_log` with `post_id = NULL` and sibling
articles' cost popovers understate their true spend.

New class `PRAutoBlogger_Research_Reaper` owns:

- `on_cron()` — WP-Cron entrypoint hooked to the daily
  `prautoblogger_reap_orphan_research_rows` event (already scheduled
  by the Activator in the previous commit at 03:15).
- `reap()` — returns a `{reaped, deleted, skipped}` summary. For each
  orphan `llm_research` row older than 1 hour (grace window for live
  pipelines):
  - `_prautoblogger_run_id` post_meta primary lookup.
  - Falls back to distinct post_ids in gen_log with the same run_id.
  - If ≥1 post found → delegate to the existing idempotent
    `Post_Assembler::amortize_research_costs()`. That method inserts
    the per-article rows and deletes the original orphan in one pass,
    so the reaper is idempotent on re-run.
  - If 0 posts found AND orphan is >7 days old → delete outright as
    sunk cost with nothing to attribute to.
  - Else → skip; try again the next day.
- Never throws. DB errors caught and logged WARNING.

New WP-CLI command `wp prautoblogger reap-research` registered in the
main loader when `WP_CLI` is defined — manual ops trigger that prints
the same summary.

Departure from the CTO's spec: the reap logic lives on
`Research_Reaper` instead of `Post_Assembler`. Placing it on
Post_Assembler pushed that file to 391 lines, over the 300-line cap.
Moving it to the thin class colocates the attribution logic with the
cron/CLI plumbing that triggers it. `amortize_research_costs()` stays
on Post_Assembler — the reaper delegates to it unchanged. Flagged for
CTO review.

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-prautoblogger.php

@@ -116,6 +116,24 @@ class PRAutoBlogger {
 
 		// Single-idea generation from the Ideas browser page.
 		add_action( 'prautoblogger_generate_from_idea', [ 'PRAutoBlogger_Ideas_Browser', 'on_cron_generate_from_idea' ] );
+
+		// v0.8.1: daily reaper for orphan `llm_research` rows (when a
+		// pipeline dies before amortize_research_costs runs).
+		add_action( 'prautoblogger_reap_orphan_research_rows', [ 'PRAutoBlogger_Research_Reaper', 'on_cron' ] );
+
+		// v0.8.1: WP-CLI manual trigger for the reaper. Only registers when
+		// WP-CLI is present; no-op in normal HTTP requests.
+		if ( defined( 'WP_CLI' ) && WP_CLI ) {
+			\WP_CLI::add_command( 'prautoblogger reap-research', static function (): void {
+				$stats = PRAutoBlogger_Research_Reaper::reap();
+				\WP_CLI::success( sprintf(
+					'Reaped %d, deleted %d (stale), skipped %d.',
+					(int) $stats['reaped'],
+					(int) $stats['deleted'],
+					(int) $stats['skipped']
+				) );
+			} );
+		}
 	}
 
 	/** Register AJAX handlers for admin actions. */