Issue #393: Deprecation helper — wrap apply_filters_deprecated() and do_action_deprecated()

Labels: dx Milestone: v3.7.0 — v4 Foundations

Body

v4.0’s #360 deprecates pre-3.0 filter hooks. WP core has `apply_filters_deprecated()` and `do_action_deprecated()` since 4.6, but the plugin doesn’t use them. Add thin wrappers with consistent message format so v4.0’s deprecations are mechanical.

Plan

Add `includes/deprecations.php` with:

```php function swh_apply_deprecated_filter( $hook, $args, $version, $replacement = null, $message = ‘’ ) { $msg = $message ?: sprintf( ‘Use %s instead.’, $replacement ?: ‘the documented replacement’ ); return apply_filters_deprecated( $hook, $args, "SWH $version", $replacement, $msg ); }

function swh_do_deprecated_action( $hook, $args, $version, $replacement = null, $message = ‘’ ) { … } ```

Acceptance

• Helpers added, PHPStan typed
• Document the deprecation policy in `docs/developer/deprecations.md`: minimum 2-minor-release window before removal, `SWH x.y` version-tag format
• PHPUnit: helper fires the deprecation notice and returns the filtered value
• No existing hooks deprecated yet (that’s v4.0’s job — this just lands the tool)

Dependencies

Unblocks v4.0 #360.