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/class-activator.php

@@ -68,6 +68,7 @@ class PRAutoBlogger_Activator {
 			'prautoblogger_niche_description'    => '',
 			'prautoblogger_target_subreddits'    => '[]',
 			'prautoblogger_monthly_budget_usd'   => 50.00,
+			'prautoblogger_per_run_cost_ceiling_usd' => PRAUTOBLOGGER_DEFAULT_RUN_CEILING_USD,
 			'prautoblogger_tone'                 => 'informational',
 			'prautoblogger_min_word_count'       => 800,
 			'prautoblogger_max_word_count'       => 2000,