feat(models): OpenRouter model picker + AJAX refresh + cost preview (v0.11.0 complete)

- AJAX endpoint 'prautoblogger_refresh_models' for manual model registry refresh
- Admin UI button on AI Models tab to trigger refresh (nonce + capability gated)
- Cost preview integration in field renderer using historical token averages
- New public method on Cost Tracker: get_avg_tokens_for_stages(array $stages, int $days = 30): array
- Enhanced field renderer with capability-specific cost calculation (outline+draft+polish for writing, single stage for analysis/editor)
- Autoloader updated to scan ajax/ directory for model registry refresh handler
- Admin JS enhanced with AJAX toggle handler for refresh button
- Template updated with refresh button + nonce in AI Models section
- CHANGELOG + CONVENTIONS + version bump to 0.11.0
- All files under 300-line cap, PHPCS-clean

Spec: https://github.com/peptiderepo/prautoblogger/issues/111
Open questions all resolved per seq-02 plan-ack.

includes/class-ajax-handlers.php

@@ -150,4 +150,13 @@ class PRAutoBlogger_Ajax_Handlers {
 		}
 		wp_send_json_success( $results );
 	}
+
+	/**
+	 * Get the shared model registry instance.
+	 *
+	 * @return PRAutoBlogger_OpenRouter_Model_Registry
+	 */
+	public function get_registry(): PRAutoBlogger_OpenRouter_Model_Registry {
+		return $this->model_registry;
+	}
 }