feat(cost): per-run cost governor — reserve-before-call enforcement

Pipeline v2 Phase 1, commit 3/4. Cost_Governor reserves each call's
worst-case estimate (prompt chars/4 + max_tokens via the #18 pricing
chain) against the run ledger row with one atomic conditional UPDATE
(the #10 lock discipline) BEFORE dispatch, then settles to actuals after
each response; failed calls release their hold. Integrated inside
OpenRouter_Provider::send_chat_completion so EVERY text-LLM call path is
guarded with zero call-site changes; the image curl_multi batch reserves
its summed estimate in Image_Pipeline before generate_image_batch.

Breach: run halted (sticky), overage recorded, Cost_Ceiling_Exception
aborts the un-dispatched call, process_next_queued_article aborts the
remaining queued articles of halted/failed runs, and the Review Queue
shows a 'Halted runs' notice (runs halted pre-post have no draft to
list). Ceiling = prautoblogger_per_run_cost_ceiling_usd setting (default
constant $0.50, snapshotted at run start, 0 = disabled). Standalone
no-run calls, the monthly cap, and the AI Gateway path are unchanged.

Under-300: provider API-key fetch+validation moved verbatim to
OpenRouter_Request_Builder::resolve_api_key().

Agent-Session: pipeline-v2-phase1-engineer-20260612

includes/admin/class-review-queue.php

@@ -51,9 +51,22 @@ class PRAutoBlogger_Review_Queue {
 		$paged = isset( $_GET['paged'] ) ? absint( $_GET['paged'] ) : 1;
 		$query = $this->get_pending_posts( $paged );
 
+		// v0.18.0 — runs halted by the per-run cost governor are routed
+		// here for human attention (they may have no draft post to list).
+		$halted_runs = $this->get_halted_runs();
+
 		include PRAUTOBLOGGER_PLUGIN_DIR . 'templates/admin/review-queue.php';
 	}
 
+	/**
+	 * Runs halted by the per-run cost governor, newest first.
+	 *
+	 * @return array<int, array<string, mixed>> Halted run rows (max 10).
+	 */
+	public function get_halted_runs(): array {
+		return PRAutoBlogger_Run_State::halted_runs( 10 );
+	}
+
 	/**
 	 * Query generated draft posts awaiting review.
 	 *