Rename to ATT Site Healthcheck (private prefix)

Wholesale rename to avoid clashes with generic 'site healthcheck'
plugin names on a target site:

  - Plugin Name:        'Site Healthcheck' → 'ATT Site Healthcheck'
  - Main file:          site-healthcheck.php → att-site-healthcheck.php
  - Plugin folder:      site-healthcheck → att-site-healthcheck
  - Admin menu slug:    site-healthcheck → att-site-healthcheck
  - Settings slug:      site-healthcheck-settings → att-site-healthcheck-settings
  - PHP class prefix:   WPH_ → ATT_HC_
  - Function prefix:    wph_ → att_hc_
  - Option / transient: wph_* → att_hc_*
  - Action/filter:      wph_* → att_hc_*
  - CSS class prefix:   wph- → att-hc-
  - Constants:          WPH_GITEA_* → ATT_HC_GITEA_*
  - Class file names:   class-wph-*.php → class-att-hc-*.php
  - Dev folder:         ~/dev/wp-healthcheck → ~/dev/att-site-healthcheck

Existing in-progress sessions on installs that had the old wph_session
option will not migrate — they were intended for dev use only and the
user has confirmed this is OK for the rename window.

Smoke-tested on testsite: classes load, 14 steps discovered, save/load
round-trip works, admin page renders with new att-hc- CSS classes.

Recovery plugin detection unchanged — that lives in wp-site-recovery
and continues to be detected by Name + Author header.

includes/steps/90-database.php

@@ -1,7 +1,7 @@
 <?php
 if (!defined('ABSPATH')) exit;
 
-return new class extends WPH_Step {
+return new class extends ATT_HC_Step {
     public function id(): string { return 'database'; }
     public function title(): string { return 'Step 9 — Database'; }
     public function sub_items(): array {