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.

templates/admin/settings-page.php

@@ -104,12 +104,18 @@ $next_run       = wp_next_scheduled( 'prautoblogger_daily_generation' );
 					<div class="ab-panel <?php echo $active_tab === $sid ? 'ab-panel-active' : ''; ?>"
 					     data-tab="<?php echo esc_attr( $sid ); ?>"
 					     role="tabpanel">
-						<div class="ab-panel-head">
-							<h2 class="ab-panel-title"><?php echo esc_html( $sec['title'] ); ?></h2>
-							<?php if ( ! empty( $sec['description'] ) ) : ?>
-								<p class="ab-panel-desc"><?php echo esc_html( $sec['description'] ); ?></p>
-							<?php endif; ?>
-						</div>
+					<div class="ab-panel-head">
+						<h2 class="ab-panel-title"><?php echo esc_html( $sec['title'] ); ?></h2>
+						<?php if ( ! empty( $sec['description'] ) ) : ?>
+							<p class="ab-panel-desc"><?php echo esc_html( $sec['description'] ); ?></p>
+						<?php endif; ?>
+						<?php if ( 'prautoblogger_models' === $sid ) : ?>
+							<button type="button" id="prautoblogger-refresh-models" class="ab-btn ab-btn-secondary ab-btn-sm" data-nonce="<?php echo esc_attr( wp_create_nonce( 'prautoblogger_refresh_models' ) ); ?>">
+								<span class="dashicons dashicons-update"></span>
+								<span class="ab-btn-label"><?php esc_html_e( 'Refresh Model List', 'prautoblogger' ); ?></span>
+							</button>
+						<?php endif; ?>
+					</div>
 						<table class="form-table ab-form-table" role="presentation">
 							<?php do_settings_fields( 'prautoblogger-settings', $sid ); ?>
 						</table>